316 PKOF. H. a. SEELEY ON SIMILITUDES OF 



have a tendency to unite mesially). The palate difters in having 

 its anterior part largely made by the vomerine bones, which 

 meet the potential presphenoid style behind. 



The premaxillary and maxillary bones usually carry a contin- 

 uous series of teeth smaller than those of Ichthyosaurus, while 

 there is an inner series of teeth (on the vomers and palatine bones) 

 of which some are larger than those of Ichthyosaurus. The ar- 

 rangement of palate-bones is very unlike the ichthyosaurian. The 

 substance of the teeth seen in transverse section is much more 

 folded than in Ichthyosaurus. In Mastodonsaurus, according to 

 Von Meyer, the presphenoid, basisphenoid, and occipital bones 

 are all differentiated, thus approximating to Ichthyosaurus, though 

 the articulation with the skull is made by two condyles. The 

 surface of the skull iTsually differs in being sculptured. 



The centrum of the vertebra is similar in the two groups, 

 being very short and biconcave ; but the neural arch is more 

 like that of a crocodile than that oi Ichthyosaurus in supporting the 

 ribs on transverse processes. The ribs are similar to those of 

 Ichthyosaurus in their length, flattened character, and the double 

 head by which they articulate with the vertebra. 



PAET II. 



THE BESEMBLANCES OE PLESIOSAUIUAN BONES TO THE 

 BONES OE OTHEB ANIMALS. 



§ 1. The Mammalian Characters of Plesiosaurus. 

 The sutures are not all well seen in skulls of Plesiosaurus ; but 

 enough is manifest to show that, in the small size of the parietal 

 region, the large temporal fossa, and the considerable develop- 

 ment of premaxiUary, maxillary, and nasal bones in front of the 

 anterior nares, the upper surface of the skull differs in aspect 

 from the mammalian type. If the orbits are circumscribed by 

 bones as they are in Euminants and many other mammals, there 

 is a difference, owing to the frontal bone being excluded, and its 

 place taken by two bones which Prof. Owen names superorbital 

 and postfrontal, but which it would be more consonant with 

 simple naming to call respectively postfrontal and postorbital. 

 The lachrymal bone does not similarly enter into the anterior 

 nares in mammals, though in certain Euminants there is so 



