in North America of rare Extinct Vertebrates. 57 



vertical extension attended with piscine confluence of the 

 hsem- (Ji) with the hyp- (y) apophyses (ib. fig. 11). 



No Mosasauroid has hitherto been discovered so entire and 

 undisturbed, or exhumed with such exact care, as to yield a 

 precise numerical vertebral formula. Even in the skeleton 

 of the Leiodon described by Prof. Snow, " only two of the 

 twenty vertebras, which were scattered over the slab in all 

 positions, remained united "*. 



The nearest approaches to the true number in the genus 

 Mosasaurus I believe to have been made by Cuvier (who assigns 

 133) and by Goldfuss (157). Of Leiodon I estimate, pro- 

 visionally, from the various data at command, the follow- 

 ing :— 



Number. 



Atlas and axis 2 



Vertebrae (type 4) with haemapophysis, hypapophysis, di- 

 apophysis, and zygapophysis 5 



„ (type 5) with hypapophysis, zygapophysis, and 



diapophysis 7 



„ (type 6) with zygapophysis and diapophysis ... . 18 



„ (type 7) with diapophysis 22 



„ (type 8) with parapophysis 15 



„ (type 9) with parapophysis and unankylosed 



haemal arch 24 



,, (type 10) with unankylosed haemal arch 14 



„ (type 11) with ankylosed haemal archf 44 



» (tjP e 12) with centrum and neurapophyses rudi- 



mental or none 7 



158 



The vertebras of Leiodon are devoid of the accessory zygan- 

 tral and zygosphenal articulations. 



All the ribs or pleurapophyses, where present or preserved, 

 are monocipital. 



In entering upon the description of the British Fossil Rep- 

 tilia 1 1 found the descriptive phrases applied in Anthropptomy 

 to the parts and processes of the vertebrse cumbrous, if at all 

 applicable to the homologous parts in those of the lower (especi- 

 ally the cold-blooded) Vertebrates, some apophyses in which 

 had no homologues in the human vertebras. I therefore pro- 

 posed a "Nomenclature," substituting " names" for "phrases," 

 and devised names for parts which previously had none. 



* Op. cit. p. 57. 



f All the above have the centrum (o) and ankylosed neural arch and 

 spine (ns). 



X " On the Plesiosaurus macrocephalm" Trans. Geol. Soc. vol. v. ser. 2, 

 p. 515 (18:38) ; " Reports on British Fossil Reptiles," in Transactions of 

 the British Association for 1840 and 1841. 



