Order III. EUSUCHIA.
Crocodilia in the stricter sense. The premaxillae are short and always enclose the nostrils. The choanae lie behind the palatines, in recent forms even within the pterygoids. They occur from the Liassic or Lower Jurassic period to the present time.
The direct ancestors of the Eusuchia are still unknown. They cannot have been developed from the Pseudosuchia, nor do we know intermediate stages which connect them with the Parasuchia. The nostrils, situated within the premaxillaries, always lie in front of the nasals, although these sometimes extend forwards and form a bony internasal septum fusing with the usual cartilaginous septum. The choanae, instead of opening immediately behind the vomer, are carried far back, owing to the formation of a secondary bony palate. In the Jurassic Crocodiles this roof is formed by the meeting of the palatine bones in the medio-ventral line, and the choanae open immediately behind. From Cretaceous times onwards this roofing is continued by the pterygoids, which likewise form a median suture; and the united choanae (which may, or may not, be divided by a thin bony septum) are pushed towards the posterior end of the pterygoids. Since the Jurassic times there exists also a tendency to enclose the Eustachian passages (the remnants of the first gill-clefts) by bone. In the earlier members they were still wide slits or open grooves on the ventral side of the basi-occipital bone. Since the Cretaceous epoch they have been transformed into bony canals and open through one median hole, situated between the basi-occipital and the basisphenoid, immediately behind the posterior symphysis of the dorsal portion of the pterygoids, which latter almost completely cover the basisphenoid. The vomer is not visible (except in Caiman niger), being covered by the ventral junction of the palatines and maxillaries. The broad, lateral wings of the pterygoids are connected by separate bones, the ectopterygoids = transpalatines = transverse bones, with the maxillaries, and in recent forms also with the jugals. Thus an extensive, very firm bony palate is produced; and the large palatal foramina, between the palatines, maxillaries, ectopterygoids and pterygoids, are closed by the same dense mucous membrane which cover the whole roof of the mouth.
The opisthotic and epi-otic bones fuse early with the lateral and with the supra-occipital bones; only the pro-otic remains longer as a separate element, perforated anteriorly by a large hole for the exit of the third branch of the trigeminal nerve. The basisphenoid is scarcely visible, being covered by the pterygoids. The presphenoid is large, continued forwards and upwards into the usually cartilaginous interorbital septum. Near the anterior and upper margin of the presphenoid is a large notch on either side for the passage of the optic nerve, the three eye-muscle nerves and the first branch of the trigeminal nerve. There are no separate orbito-sphenoids, their place being taken by membrane or cartilage in continuation with the interorbital septum, but the alisphenoids are large, abutting upwards against the frontals. Each prefrontal sends down a vertical process which joins the palatine of its side.
The configuration of the snout varies much. There are two parallel lines of development since the Jurassic epoch, namely, long-snouted creatures, of which two still survive as Gavialis and Tomistoma, and more broad and short-snouted members like the rest of the Crocodiles and Alligators. In opposition to the Parasuchia the elongation of the snout is effected by the maxillaries. The length of the nasals varies much, mostly in conformity with that of the maxillaries. As a rule they reach the premaxillaries but not always the nasal groove. In Gavialis they are short, far separated from the premaxillaries by the maxillaries, which meet in the dorso-median line. The orbit is bordered by the frontals, which at an early age fuse into an unpaired piece, and by the prefrontal, lacrymal, jugal, and postfrontal. At a deeper level the orbit is partly divided from the lateral temporal fossa by a strong column which is formed by the meeting of a downward process of the postfrontal with an inner process of the jugal, and an ascending process of the ectopterygoid (cf. Fig. 108, p. [458]). This arrangement adds considerably to the strength of the skull. The lateral temporal fossa is bordered in front by the column just described; below by the jugal and the quadrato-jugal, which is firmly wedged in between the jugal and quadrate; behind by the quadrate; above by the postfrontal, which forms a strong superficial bridge with the squamosal. This rests upon and often fuses with the quadrate and an intervening transverse wing-like extension of the lateral occipital bone. By this squamoso-postfrontal bridge part of the original temporal fossa is divided into the lateral one just described, and a dorsal fossa. The latter is bordered by the postfrontal, squamosal, and united parietals. This dorsal temporal fossa is consequently not homologous with that of the Parasuchia, a vestige of which is however present in many, especially in young skulls of Crocodiles, in the shape of a narrow passage which extends backwards from the dorsal fossa, bridged over by the junction of the parietal with the squamosal, and bordered below by the occipitals.
The size of the upper temporal fossae stands in an inverse ratio to that of the lateral fossae. In the older Eusuchia the upper were the larger of the two. The temporo-mandibular muscle which lifts or shuts the lower jaw arises from the walls of the upper fossa, passes beneath the jugal arch, and is inserted into the supra-angular portion of the lower jaw. In the more recent Crocodiles this muscle is more and more superseded by the pterygo-mandibular muscle, which, arising chiefly from the dorsal surface of the much broadened-out pterygoid bone, fills the widened space between the latter and the quadrate, and is inserted into the outer surface of the os angulare of the lower jaw. This muscle, owing to its general disposition, is capable of much more powerful development and leverage than the temporo-maxillary muscle, which latter, being more reduced, allows the dorsal fossae to be more and more closed up by the surrounding bones.
The fossae are still comparatively large in the long-snouted genera Gavialis and Tomistoma, which live entirely upon fish and scarcely chew their food, whilst these holes almost completely disappear in some of the Alligators, namely in the broad- and short-snouted members, which, having a varied diet, taken from every available group of the animal kingdom, chew their prey.
The quadrate extends obliquely backwards, and is immovably wedged in and partly fused with the quadrato-jugal, the squamosal, and the lateral occipital wings. Between the latter and the quadrate remains a slit-like canal, well visible from behind, through which passes the continuation into the mandible of the columellar or ossicular chain of the auditory apparatus. Intricate passages, used as additional enlargements of the space of the middle ear, pervade the proximal portions of the quadrate and the roof of the cranium beneath the parietal bridges mentioned above, the two sides communicating with each other. The supra-occipital bone is visible from behind; its top is covered and partly fused with a continuation of the parietals, which are, like the frontals, fused into an unpaired mass. The occipital condyle is formed entirely by the basi-occipital bone, so far as the articulating facet is concerned, but it is supported on either side by a lamella from the lateral occipitals.
The two halves of the lower jaw form a symphysis of very variable length. Each half is composed of six bones. (1) The articulare, perforated in its upper, posterior, inner corner by a canal for the reception of the siphonium, a narrow tube of connective tissue, which connects the cavities of the middle ear with the large empty space enclosed within the lower jaw; (2) the angulare; (3) the dentary, which alone carries the teeth; (4) the splenial, a long splint-like bone on the surface of the inner or median side of the jaw, of variable length; (5) the operculare, the counterpart of the splenial on the outer side; (6) the supra-angulare, which forms the dorsal border of the lower jaw between the dentary and the angulare.
The teeth, which are more or less conical or compressed laterally, are deeply implanted in separate sockets. They are often shed throughout life, the successors lying on the median side, and with their caps partly fitting into the wide, open roots of the teeth to be expelled. The number of teeth in the premaxilla is universally five on either side in recent forms, but in a few species, e.g. Crocodilus niloticus and C. porosus, the second pair is lost with maturity and is not replaced. In the broad-snouted kinds, especially in the Alligators, most of the upper teeth overlap laterally those of the lower jaw. In most species of Crocodilus the overlapping is less marked and the teeth partly interlock, but the fourth mandibular tooth, generally the strongest and longest, is received into a lateral notch at the junction of the premaxillary and maxillary. Frequently those of the longer lower teeth which fit into pits of the upper jaw, gradually transform the pits into holes by continued pressure upon the bone, and in old specimens the tip of the lower tooth may even perforate and stand out above the skin of the snout.