PLESIOSATKIDJE. 



123 



It may be observed that in young individuals the terminal faces 

 of the cervical vertebrae are much natter than in the adult ; and the 

 ischia (fig. 44) are less elongated. 



There has been considerable confusion as to the species which 

 should be regarded as the type of this genus. Thus on page 51 of the 

 'Proc. Geol. Assoc' for 1883 it is stated by Hulke that the genus 

 was formed by Owen in the ' Eep. Brit. Assoc' for 1841, pp. 60-G3 

 (1842), for the reception of the two species originally designated by 

 him Plesiosaurus grandis and Plesiosa urns trochanterius. A reference 

 to pp. 04, 65 of the ' Rep. Brit. Assoc' torn. cit. shows, however, that 

 this is a misapprehension, and that the type is undoubtedly the im- 

 perfect skeleton in the Oxford Museum from the Kimeridge Clay of 

 Market-Bason, Lincolnshire, originally described under the name of 

 Pliosaurus brachydirus in Owen's ' Odontography,' pt. ii. (1841). 

 Owen himself has, however, shifted his ground, since, in his ' Fossil 

 Beptilia of the Kimeridge Clay ' (Mon. Pal. Soc), pt. iii. p. 6 (18G9), 

 he takes the name Pliosaurus grandis for the type of the genus, and 

 suggests that P. hracliydirus is merely a variety of the same. This 

 departure from the original view is, however, inadmissible ; and there 

 is, moreover (as Hulke has pointed out), no evidence to show that 

 the humerus or femur on which Plesiosaurus grandis was founded 

 does really belong to that genus. 



Pliosaurus brachydirus, Owen 1 . 



Syn. Plesiosaurus giganteus, Conybeare 2 . 



The type species. Considerably smaller than P. macromerus. 

 Teeth (fig. 37) with the carina) strongly developed, and the intcrcarinal 

 space large and flat, without any trace of vertical ridges ; 35 teeth 

 in lower jaw, and mandibular teeth increasing in size till the 12th 

 cervical vertebras without bevelled edges to the terminal faces of the 

 centra, with the costal facets moderately prominent, unequal, and not 

 deeply excavated ; and in the anterior region with a mammilla 

 surrounding the central puncture ; texture of bone very tine and 

 smooth. Humerus and femur with small proximal trochanter ; 

 femur with the shaft suddenly contracted in the middle, and ex- 

 panding gradually at both extremities. 



The dimensions of one of the later cervical vertebra) of the type skele- 

 ton are:— length 0,041 (1-6 inches), height 0,086 (3-1 inches), mid 

 width 0,000 (3-0 inches) ; the length being less than half the width. 



All the known examples are from the Kimeridge (lav. 



11(d). Europe (England). 



1 Odontography, pt. ii. p. 283 (1841). 



2 Trans. Geol. Soc. scr. 2, vol. i. pt. ii. p. 080 (1824).— Insufficiently charac- 

 terized. 



