460 MESOPTERYGOID IN CERTAIN REPTILIAN SKULLS. 



Crossopterygians. Now, however, this difficulty has been removed 

 by the descri})tion of the palate of Eusthenopteron foordi by 

 Bryant*, and we have the clue to the little element in both 

 Gonodectes and the lizards (text-fig. 7). As will be seen from the 

 fio-ure, there is a large bone which has on its outer side a palatine, 

 and an ectoj^terygoid, and which is manifestly the pterygoid of the 

 higher forms. Internally it articulates with a long, slender bone 

 which is the mesopterygoid of the bony fishes. This mesoptery- 

 goid is supported by the parasphenoid or vomer. The relations 

 of all the bones is very similar to what we have in the higher 

 forms, and the only striking difference is that in the Tetrapods 

 the mesopterygoid has become greatly reduced and usually lost. 

 But when it is retained it is found, as in the E,hipidistian fish, 

 suppoi'ting the pyterygoid and itself articulating with the basi- 

 pterygoid process of the basisphenoid or the parasphenoid. 



There is one other group of fossil reptiles in which it is 

 possible there is also a distinct mesopterygoid. Sollast, in 

 describing recently the skull of Ichthyosauo^us from serial sections, 

 showed that there are two epipterygoid-like elements on each 

 side. The posterior pair are, I think, manifestly the true epi- 

 pterygoids. Not improbably the anterior pair will prove to be 

 mesopterygoids, which in the specimen examined by Sollas had 

 been slightly displaced outwards by crushing. 



* "On the structure of JSusthenopteron " Bull. Buffalo Soc. Nat. Sci. xiii. 

 pp. 1-59 (1919). 



t "The skull of Ichthyosaurus, studied in serial sections," Phil. Trans, vol. 208 B 

 (1916). 



