Page:The Osteology of the Reptiles.pdf/41

Rh animal. Primitively more or less excluded from the margin of the foramen magnum.

Only in certain plesiosaurs is the supraoccipital paired, by the extension of the large foramen magnum to the parietal roof. In most reptiles save the Ophidia and Crocodilia, it enters more or less into the boundary of the foramen magnum.

Exoccipitals (eo). Primitively (Figs. 21, 42 ) small, forming the larger part of the boundary of the foramen magnum, approximated to each other both above and below, closely articulated with the basioccipital only.

Primitively the exoccipitals took but little part in the formation of the occipital condyle, but in many later reptiles they form a large part, as in the Chelonia (Fig. 31 ), or even the whole, as in the Amphisbaenia (Fig. 56 ); or, by the recession of the basioccipital, the double condyles of the Cynodontia and mammals.

Paroccipitals (po). (Figs. 9, 21 .) Only in the Cotylosauria primitively do the paroccipitals exist as distinct bones in the adult, articulating with the exoccipitals, supraoccipital, proötics, stapes, tabulars, and quadrates. On the inner side they help form, with the supraoccipital and proötics, the otic capsule. In the Theromorpha, so far as known, the paroccipitals are fused with the supraoccipital, suturally or loosely articulated with the exoccipitals. In the Chelonia (Fig. 31, op), only of modern reptiles, are they separate bones in the adult, intercalated between the exoccipitals, supraoccipital, proötics, squamosal, and supporting the head of the quadrate. Among other reptiles they are known to be free only in the Ichthyosauria (Fig. 51), articulating with the basioccipital, exoccipitals, stapes, and so-called supratemporal. In other reptiles they are indistinguishably fused with the exoccipitals in the adult.

Proötics (pc). The proötics (Figs. 8, 10, 11, 30, 59, 69) are a conspicuous part of the brain-case, intercalated between the supraoccipital, paroccipitals, basioccipital, basisphenoid, and, when present, the postoptics, and containing a part of the internal organ of hearing. Their relations are yet poorly known in the primitive reptiles. They usually have foramina perforating them for the passage of the third and sixth nerves, and form the posterior boundary of the foramen for the fifth nerve; posteriorly for the eighth, ninth, and tenth nerves. They form a large part of the brain-case exteriorly in