62 
SELACHII. 
Petalodontidsc. The two described species are from the Lower Car- 
boniferous of Russia, and named Oymatodiis plicatu.lus,Tra,\it»a)^o\A, 
Nouv. Mem. Soc. Imp. Nat. Moscou, vol. xiv. (1879), p. 53, pi. vii. 
fig. 3, and G. reclinatus, Bull. Soc. Imp. Nat. Moscou, 1883, pt. ii. 
p. 109, pi. V. figs. 3, 4. The tooth is very much compressed, and the 
coronal margin wavy. 
Family PRISTODONTID^. 
An indefinable e.xtinct family, known only by detached teeth, of a 
type very similar to some of those referred to the Petalodontidae. 
Each tooth is bilaterally symmetrical, and the coronal contour of 
one is hollowed in such a manner as to precisely “ fit ” the crown of 
the other tooth directly opposed to it. These characters are 
suggestive (though not conclusive proof) of there having been but 
a single tooth in each jaw of the original fish. 
Genus PRISTODUS, Davis (ex Agassiz, MS.). 
[Trans. Roy. Dublin Soc. sor. 2, vol. i. 1883, p. 519.] 
Syn. Diodontopsodm, J. W. Davis, Brit. Assoc. Rep. 1881, p. 640.. 
Crown of tooth comparatively thin and plate-like, vertical in front, 
but sharply bent backwards at a short distance below the apex, 
thus forming a posterior horizontal portion. The latter portion is 
flat, with an excavated hinder border, and the vertical portion rises 
abruptly from its semicircular front margin, with a sharp cutting- 
edge, highest in the middle and gradually becoming less elevated 
on each side. Root short and thick, deepest in front, fixed to the 
horizontal portion of the crown, immediately behind the anterior 
margin (PL I. fig. 13). 
In the tooth of one jaw the crown is much thickened at its 
flexure, and thus, though appearing sharply bent from the anterior 
asi)cot, slopes in a gradually ouired plane on the posterior face 
(PI. I, fig. 13). The directly opposing tooth “ bites ” outside this 
one, and accordingly there is a well-marked groove upon its pos- 
terior face at the boundary of the sharply separated vertical and 
horizontal moieties of the crown, the groove becoming gradually 
deeper to a pit in front which receives the opposing apex. There 
is no evidence as to the precise relations of these two forms of 
teeth, but, for convenience of reference, the first may be termed 
lower, the second upper. 
As already recognized by William Davies, R. Etheridge, jun., and 
