12 FOSSIL REPTILIA OF THE 



Dorsal centrums are usually the least significant of specific characters, owing 

 to the limitation of the articular surfaces to the neurapophysial and terminal ones, 

 and also owing to a resumption, more or less, in the dorsal region of the more 

 common proportions of the centrum, when this is departed from, either in excess 

 of breadth, depth, or shortness, in the cervical region. 



Dr. Campiche's description is so minute and exact that the correspondence of 

 the dorsal centrum (Tab. VI, figs. 9, 10, 11) with the characters expressed at p. 43, 

 op. cit., and shown in " Plate VI " of that work, will be found to justify the specific 

 approximation. The centrum of PI. neocomiensis is " a little broader than high, so 

 that the articular surfaces form nearly a transverse, very slightly elongated, 

 ellipse ; the shape would be even better expressed by a circle, of which the upper 

 part was flattened and subtruncate (see fig. 18)."* " The length is sensibly inferior 

 to the two other dimensions ; the sides are strongly and gradually excavated, so that 

 when the vertebra is viewed from above," (as in fig. 11, Tab. VI) op. cit., "its middle 

 part is much narrower than its articular surfaces. The inferior region, corresponding 

 to the medial line of the body, is more feebly excavated.! The two large and deep 

 neurapophysial pits are slightly arched inwardly, and are two and a half times as 

 long as they are large ; but the most significant character is the slight concavity 

 of the terminal surfaces, with their middle part feebly raised into an irregular 

 protuberance." 



In the larger of the dorsal centrums from the Swiss Neocomian, measuring 

 2 inches 7 lines in transverse diameter, the mediau rising is 10 lines in diameter, but 

 not more prominent than the more circumscribed rising in Tab.VI, fig. 10 of the pre- 

 sent Monograph. In the smaller Swiss centrum (Plate VI, fig. 2 of op. cit.) the central 

 eminence is broader and lower than in the nearly equal-sized centrum (Tab. VI, fig. 

 10) of the present Monograph ; nevertheless, I am inclined to think that the mam- 

 millate character of the terminal articular surfaces shown in the cervical vertebra? 

 may, like other characteristic modifications, be less strongly manifested in the dorsal 

 vertebrae, or in some of the dorsal vertebrae of the same individual ; and, therefore, 

 I supersede my MS. denomination of Plesiosaurus mamillatus, under which I dis- 

 tinguished those vertebrae from the Cambridge Green-sand, when first obtained 



* " Un peu plus larges qu'ils ne sonthauts, en sorte, que leurs faces articulaires forment, apeupres, une 

 ellipse transverse tres peu allongee. Leur forme serait meme mieux exprimee par un cercle dont la partie 

 superieure serait aplatie et subtronqu£e." — Op. cit., p. 43. 



f " La longeur est sensiblement inferieure aux deux autres dimensions. Les flancs sont fortement et 

 graduellement excaves, en sorte que, lorsqu'on regarde la vertebre au dessus, sa partie mediane est beau- 

 coup plus etroite que les faces articulaires." "La region inferieure qui correspond a la ligue mediane des 

 corps est beaucoup plus faiblement inflechee. A la face superieure, on voit deux grandes et profondes im- 

 pressions, correspondant a l'insertion des neurapopbyses ou lames tectrices. Elles sont un peu arquees 

 en dedans, deux fois et demie aussi longues que larges, les faces articulaires sont legerement concaves, avec 

 leur milieu faiblement releve en une protuberance irreguliere." — Op. cit., p. 43. 



