194 BRITISH FOSSIL REPTILES. 



organised according to the type of the existing Lacerfilia and not of the Enaliosauria 

 or Cetacea. But a foot so organised for crawling on land might, nevertheless, by the 

 webbed union of the large and long unguiculate claws, have been well adapted, like 

 the feet of the Amblyrhynchus and Alligator, for swimming ; and the modifications of 

 the vertebral column, especially of the long and deep tail of the Mosasawr, clearly 

 prove it to have been more strictly aquatic in its habits than any known existing 

 lizard.* 



The vertebra from the Chalk near Lewes (PI. 1, figs. 1 and 2) above alluded to, 

 which is the subject of the cut. No. 2, p. 146, of Dr. Mantell's 'Geology of the South- 

 East of England,' is one of those posterior dorsal or lumbar vertebrae, in which the 

 diapophysis {d) arises from near the middle of the side of the centrum, and has a 

 depressed flattened form, at its origin, instead of the thicker subcompressed form that 

 characterises the same process in the anterior dorsal vertebrae. The specimen in 

 question is much mutilated ; both the neurapophyses, 7i, the diapophyses, d, and part 

 of the left side of the centrum, are broken away ; but the rarity of such evidences 

 of the Mosasaurian genus in our English Chalk, and the historical interest attached to 

 this, which is one of the first specimens discovered, has induced me to give an 

 accurate figure of it in PI. 1, fig. 4, together with one of the homologous vertebrae 

 of the Macstricht species (fig. 4), which is preserved in the British Museum. The 

 specimen from Lewes presents the following dimensions : — 



Inches. Lines. 



Length of the centrum 2 



Vertical diameter of ditto ....... 1 4 



Transverse diameter of ditto ....... 1 6 



Length of the base of the neural arch ..... 1 8 



The neural arch, n, has completely coalesced with the centrum : it terminates 

 behind, about four lines from the convex articular end of the centrum. The marginal 

 circumference of that surface, fig. 2, has been worn away, but it evidently presented 

 a more obovate and less triangular figure than in the Mosasaurus Tloffmanm, fig. 5. 

 The fractured base of the diapophysis, shown at (/, fig. 1, is situated lower than half- 

 way down the side of the centrum. 



The two caudal vertebrae (PI. I, fig. 3) have been retained in natural juxtaposition 

 in the same block of Chalk. Both the neural (») and haemal (A) arches have coalesced 

 with the centrum without any trace of the primitive sutures, the antero-posterior 

 extent of the neurapophysis is relatively shorter than in the more advanced vertebra, 



* M. Hermann von Meyer, in his comprehensive and useful summary of Fossil Eemains, entitled 

 ' Palseologica,' 8vo, 1832, classifies the Mosasaurus with the Plesiosaurus, in the Order of Sauria, 

 characterised by fins. ("Saurier mit flossartigen Gliedmassen," p. 201.) 



