THE SKULL OF REPTILES 23 



animal. Primitively more or less excluded from the margin of the 

 foramen magnum. 



Only in certain plesiosaurs is the supraoccipital paired, by the ex- 

 tension 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 b, 42 d) 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 b), or even the whole, as in the Am- 

 phisbaenia (Fig. 56 b) ; or, by the recession of the basioccipital, the 

 double condyles of the Cynodontia and mammals. 



Paroccipitals (po). (Figs. 9, 21 b.) Only in the Cotylosauria primi- 

 tively do the paroccipitals exist as distinct bones in the adult, articu- 

 lating with the exoccipitals, supraoccipital, prootics, stapes, tabulars, 

 and quadrates. On the inner side they help form, with the supra- 

 occipital and prootics, the otic capsule. In the Theromorpha, so far 

 as known, the paroccipitals are fused with the supraoccipital, sutur- 

 ally or loosely articulated with the exoccipitals. In the Chelonia 

 (Fig. 31 B, op), only of modern reptiles, are they separate bones in 

 the adult, intercalated between the exoccipitals, supraoccipital, 

 prootics, squamosal, and supporting the head of the quadrate. 

 Among other reptiles they are known to be free only in the Ichthy- 

 osauria (Fig. 51), articulating with the basioccipital-, exoccipitals, 

 stapes, and so-called supratemporal. In other reptiles they are indis- 

 tinguishably fused with the exoccipitals in the adult. 



Prootics (pc). The prootics (Figs. 8, 10, 11, 30, 59, 69) are a con- 

 spicuous part of the brain-case, intercalated between the supra- 

 occipital, 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 pas- 

 sage 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 



