1858.] HUXLEY — PLESIOSAURUS. 293 



4. Contrary to what is commonly stated, the post-frontal appears 

 to me, in P. Etheridgii and Hawkinsii at any rate, to articulate with 

 a bone, the homologue of the squamosal* of the Crocodile. 



5. The squamosal of the Plesiosaurus seems to have been con- 

 founded by some with a process of the parietal. 



6. The temporal fossa is divided by the post-frontal in the manner 

 so characteristic of, though not absolutely peculiar to, the Croco- 

 dilian reptiles. The great superior fossse correspond with the large 

 superior temporal fossae of the Teleosaurian, and even the narrowness 

 and crested form of the upper surface of the parietal (supposed to 

 be distinctive of the Plesiosaurus) are very closely approached in 

 such Teleosauria as T. temporalis. 



7. The exoccipital sends outwards and downwards a process which 

 reaches the great quadratum, and between this below, the quadratum 

 externally, and the squamosal above, there is a large aperture in the 

 Plesiosaurus. 



In the triassic Enaliosauria, however, the corresponding interval 

 is, judging by Hermann von Meyer's figures of Nothosaurus, smaller 

 in proportion, or, as in Simosaurus, absent (?). On the other hand, 

 it is larger in the Teleosauria than in the existing Crocodilia. 



8. The basi-sphenoid appears upon the base of the skull for a 

 great space in the Plesiosaurus, while in the ordinary Crocodile it is 

 not visible at all, being hidden by the pterygoids. Even in the 

 Gavial, however, the basi-sphenoid shows itself fully on the base of 

 the skull, while in the Teleosaurus it is as much exposed as in the 

 Plesiosaurus, and presents a median ridge with a deep fossa on 

 either side. There are a similar ridge and fossae in Plesiosaurus, the 

 latter having, I imagine, been mistaken for the posterior nares. 



9. I believe the posterior nares were situated far forwards in the 

 Plesiosaurus ; for in the first place, the object of their being situated 

 as in the Crocodile is not intelligible teleologically ; and on morpho- 

 logical grounds we should expect to find them anteriorly situated, 

 for the Gavial has them more forward than the Crocodile, and the 

 Teleosaurus than the Gavial. In the latter, indeed, they are so far 

 forward, that the pterygoids do not enclose them below at all. They 

 are nearly on a line with the middle of the orbits. 



10. The pterygoids of the Plesiosaurus send processes backwards 

 to abut against the quadratum f. The ends of the corresponding 

 bones are broken off in the Teleosauria I have examined. 



11. The descending process of the basi-occipital is single in the 

 ordinary Crocodile, but in the Teleosaurus it is divided into two 

 widely separated tubercles as in Plesiosaurus. 



12. The supra-occipital is widely separated from the edge of the 

 occipital foramen by the exoccipitals in the Crocodile. In the Teleo- 



* This is the hone commonly but erroneously termed the " mastoid " in the 

 Crocodile. Rathke and Hall man have long since satisfactorily shown that the 

 homologous bone has no relation with the true mastoid. 



f These processes are particularly well shown in the very instructive specimen 

 labeled 14,550 in the British Museum. I propose to figure this as well as some 

 others in my Memoir. 



