Fig. 73.–Sphargis coriacea, the "Leathery Turtle," young specimens, ventral and dorsal views. × 1.
According to Agassiz it breeds regularly every year in the spring on the Bahamas, on the Tortugas, and on the coast of Brazil, depositing its many eggs on the sandy shore like other turtles. Accidentally it visits the northern coast up to Long Island, and specimens, perhaps carried with the Gulf Stream, have been caught on the coasts of Europe, for instance off Dorsetshire. One was caught near Nantes in 1729, and is said to have made a terrible noise when being killed. This is perhaps the reason why Merrem in 1820 invented the generic name Sphargis, supposed to be derived from σφαραγέω (I make a noise). It has also been recorded from the Mediterranean. It seems to be entirely carnivorous, living upon Molluscs, Crustacea, and fish. The flesh is supposed to be unwholesome. It is a very curious fact that of this rare species only large specimens, besides a very few baby-turtles, are known or preserved in collections, while individuals of intermediate size, say from four inches to three feet in length, have never been recorded. If it were not for the fact that they are still known to breed, it would look as if the species were dying out. Perhaps they are very shy, leading a pelagic life, diving at the least sign of danger, and coming near the land only for the sake of breeding.
The structure of Sphargis is so peculiar in many respects that it deserves a somewhat full account. The neuro-central sutures persist on all the vertebrae. The eight cervicals are short. All the ten trunk-vertebrae carry ribs, and these, with the exception of the last, articulate between the centra and with the neural arches; the first and tenth ribs are short, the others are long and flattened, but not broad, with wide spaces between them. The tail is short, although it consists of about twenty vertebrae; these are devoid of chevrons.
The skull superficially resembles that of Chelone, chiefly owing to the completely roofed-in temporal region. The supraoccipital crest is rather short, covered completely by the parietals, the posterior margin of which is rounded off instead of forming, as in the Chelonidae, a long projecting triangular crest with the supra-occipital. The parietals are in broad contact with the postfrontals, posteriorly they are just reached by the squamosals. The quadrato-jugal is small, separated from the postfrontal by the meeting of the squamosal with the jugal. The quadrate is notched behind, and it separates the opisthotic from the squamosal. The basisphenoid is large and broad, extending far forwards so as to separate the pterygoids widely from each other except in their anterior portions, which, instead of sending a lateral arm to the jugal and maxillary, as in Chelone, are widely separated from these bones by the palatines. The choanae lie on either side of the anterior half of the vomer, and are not roofed over by ventral vomero-palatine wings.
The limbs and their girdles are essentially like those of the Chelonidae, but are not derivable from them. The most remarkable feature is the shell. The dorsal and ventral halves are directly continuous, forming one unbroken case all round, which is composed of many hundreds of little bony plates, irregularly polygonal, fitting closely into each other with their sutural edges, and giving the shell a beautiful mosaic appearance. On the dorsal side are a median row and three pairs of lateral rows of larger plates, and these form seven longitudinal blunt ridges which all converge towards the triangularly pointed tail-end of the shell. The ridges are not so much produced by thickened or spine-like edges of the plates, but by the right and left halves of the plates being actually bent at an angle. This is most conspicuous at the sides of the shell where it passes into the ventral portion. The latter has two pairs of lateral and one median ridge. The whole shell has consequently twelve ridges. The mosaic plates are deeply imbedded in the cutis, being externally as well as internally covered or lined with dense leathery skin. The epiderm is thin, and shows no indications of horny scales. In young specimens the whole shell is soft and very imperfectly ossified, later on it is quite rigid, although comparatively thin. It is nowhere in contact with the internal skeleton, except by a nuchal bone, which by a descending process articulates with the neural arch of the eighth cervical vertebra.
The affinities of the Sphargidae and their position in the system are still debatable. Whilst some authorities, e.g. Cope, Dollo, and Boulenger look upon Sphargis as the sole remnant of a primitive group in opposition to all the other recent Chelonia, Baur considered it the most specialised descendant of the Chelonidae. Dames agreed with him. Van Bemmelen has modified this view in so far as he regards Sphargis as the most specialised Chelonian, but considers the differences between it and the Chelonidae great enough to conclude that both Sphargidae and Chelonidae represent two independent, partly parallel, branches which have arisen from two different groups of terrestrial tortoises. Case,[[130]] from the study of Protostega and other fossil forms, tends towards Baur's view. He believes that Sphargis is the culminating form of a branch which through Psephophorus and with Eosphargis has sprung from some creature like Lytoloma, which at the same time is the starting-point of another branch which culminates in the genera Thalassochelys and Chelone, while lastly a third branch contains Protostega, Protosphargis, and Pseudosphargis. In other words, he considers them all Chelonidae. If he is right we have of course no business to separate Sphargis with its fossil allies from the rest of the Chelonia as "Athecae."
However, Case has not proved his point. It is easy enough to understand that the characters of the cranium and plastron of Sphargis are in a condition which by partial reduction can be derived from that of typical Chelonidae. The structure of the cervical vertebrae, the absence of the marginal plates and the peculiar articulation of the nuchal with the last cervical vertebra can be explained as convergent analogies, just like the paddles of Carettochelys. But the shell of Sphargis is fundamentally different from and not homologous with that of the others. Cope was therefore quite justified in distinguishing the Sphargidae as "Athecae" in opposition to the others which Dollo later on, by contrast, named "Thecophora." Unfortunate names, since both groups are undeniably in possession of a θήκη or shell. Both authors meant, however, by Theca the epidermal shields, but even this distinction is rendered invalid by Carettochelys.
The most reasonable explanation has been suggested by Hay.[[131]] The mosaic polygonal components of the shell of Sphargis are, so to speak, an earlier generation of osteodermal plates than the later generation of longer and broader bony plates which in the Thecophora come into contact, and fuse with, the neural arches and ribs. The osteoderms of Sphargis belong to the same category as the dermal ossifications in the scutes of Crocodilia, whilst the plates of the carapace and plastron of the Thecophora belong to the category of the abdominal ribs. Sphargis has the first kind in its peculiar shell, the second kind in the deeper lying plastron and in its nuchal plate. But it has lost, or perhaps had never developed, the horny shields. The only difficulty is, however, the presence of a plastron and of a typical neural plate in Sphargis. This difficulty is not very serious. The plastron is a very old institution. It occurs together with the more superficial osteoderms in Caiman, and the nuchal plate may be the oldest of all dorsals. We can scarcely imagine that the direct ancestors of Sphargis had developed both kinds of shells, and that comparatively recently the inner shell of the carapace was lost, leaving only the nuchal plate. Fossils do not support such an assumption. Undoubted ancestral forms of Sphargis are very rare. Psephophorus of the Oligocene and Miocene of Europe had a continuous mosaic shell much resembling that of Sphargis; Eosphargis is represented by a well-preserved skull from the London clay. Then follows a wide gap until we come to Psephoderma of the Rhaetic, or Upper Trias of Bavaria; the large fragment of whose dorsal shell is composed of about 200 mosaic pieces. If this fragment really formed part of the shell of a Chelonian, its age would speak greatly in favour of the Athecae being a very primitive and independent group.
Order II. THECOPHORA.