([Fig. 21]). As will be seen by the figure, practically all of the bones visible in this view have already been described, except those of the mandible, which will be described separately. At the base of the skull are, however, two bones, the basi- and alisphenoid, that have not been described and that show as well in this as in any other view. The basisphenoid (just below v and hidden by the pterygoid) was mentioned in connection with the quadrate, with whose posterior margin it articulates. It is an unpaired bone of very irregular shape. Anteriorly it is flattened out to form the rostrum, a rectangular process that forms the posterior part of the interorbital septum; in fact it is the only part of the septum present in a prepared skull, since the rest is cartilaginous. Dorso-laterally the basisphenoid articulates with the alisphenoid; posteriorly with the basioccipital; ventrally with the pterygoid; and posteriorly with the exoccipital and basioccipital. On the dorsal surface of the basisphenoid is the pituitary fossa, not seen, of course, in this view of the skull.

The alisphenoids (crossed by the dotted line from V) are a pair of very irregular bones that form most of the antero-lateral walls of the brain case. They articulate dorsally with the parietal, frontal, and postfrontal; ventrally with the basisphenoid and pterygoid; and posteriorly with the quadrate and some of the otic bones not visible in this view. Between it and the quadrate, plainly visible in this view, is a large opening, the foramen ovale (V), through which, according to Reynolds, the trigeminal nerve passes. In the middle line, directly under the frontal bone, is an opening between the anterior wings of the two alisphenoids, for the exit of the optic nerves. Ventrad and caudad to this opening, and sometimes continuous with it, is another large foramen, just dorsad to the rostrum, for the exit, according to Reynolds, of the oculomotor and abducens nerves. Projecting caudad is seen the rounded condylar part of the basi-occipital (14) to be described later, and dorso-cephalad to this is a part of the exoccipital (13) in which four foramina may be seen; of the dorsal three the one nearest the condyle and foramen magnum is for the exit (Reynolds) of the hypoglossal nerve (XII); slightly dorso-cephalad to this is one for the vagus nerve (X); between these two is a very small one for a vein; the largest and ventrally located foramen is for the entrance of the internal carotid (15). Another large foramen in the exoccipital bone will be seen and described in connection with the posterior view of the skull. Dorsal to the quadrate and largely bounded by it is the wide external auditory meatus (16), which leads into the tympanic cavity. This cavity is complicated by a number of canals that lead from it in various directions. Overhanging the cavity and meatus is the squamosal bone, described in connection with the dorsal aspect of the skull.

Fig. 21. Lateral View of the Skull of an Alligator (Caiman latirostris). ×¹⁄₃. (Brit. Mus.)
(After Reynolds.)

The Posterior Aspect

([Fig. 22]). Most of the bones seen in this view have already been described. The pterygoids (10) form the two prominent, ventro-lateral projections, while dorsal to these is the large process formed by the quadrate (8) and quadratojugal (12). The dorsal margin is formed by the edges of the parietal (23) and the squamosals (7). Immediately below the parietal is the supraoccipital (26); it is a small, triangular bone, articulating above with the parietal and squamosals, below with the exoccipitals, and anteriorly with the epiotic. It takes no part in the formation of the foramen magnum.

The exoccipitals (13) form the entire boundary of the foramen magnum except the narrow ventral portion formed by the basioccipital. Each exoccipital is a wing-shaped bone, articulating dorsally with the squamosal and supraoccipital, ventrally with the quadrate, basioccipital, and basisphenoid, and anteriorly with the opisthotic. It is pierced by five foramina, four of which were described in connection with the lateral view. Some distance laterad and somewhat dorsad to the pair already described is the fifth and largest foramen (VII); it really lies between the exoccipital and quadrate, but the former bone forms almost its entire boundary; through it, according to Reynolds, pass the seventh nerve and certain blood-vessels.

Fig. 22. Posterior View of the Skull of A. Mississippiensis.