COCHLIODOtrilD^. 
193 
As already remarked by St. John and Worthen *, the known dental 
plates of each species arc of two forms — the one, presumably per- 
taining to the upper jaw, but little inroUed and very narrow antero- 
posteriorly, tho other considerably iuroUed and of the reverse 
proportions. 
So far as known, Xystrodus may bo regarded as closely allied to 
Tomodus, the dental plates only differing from the small plates of 
tho latter genus in their less curvature and in tho peculiar arrange- 
ment of the minute coronal punctations. 
Xystrodus striatus (M‘Coy). 
184.3. CocUiudtts siriattis, L. Agassiz, Poiss. Foss. vol. iii. p. 174 (name 
only). 
1856. Cochliodus striatus, F. M'Coy, Brit. Palasoz. Foss. p. 024, pi. 3 1 . 
^ fig. 27. 
1802. Xystrodus striatus, Morris & Roberts, Quart. Journ. Geol. Soc. 
vol. xviii. p. 101 (name only). 
1862. Xystrodus anyustus, Monis & Roberts, tom. cit. p. 101 (name 
only). 
1883. Xystrodus striatus, J. W. Davis, Trans. Roy. Dublin Soc. [2J 
vol. i. p. 448, pi. liv. figs. 7-10. 
1883. Xystrodus angustm, J. W. Davis, tom. cit. p. 449, pi. Iv. 
figs. 19-21. 
1883. Xystrodus striatus, St. John & Worthen, Pal. Illinois, vol. vii. 
p. 183. 
rype. “ Upper ” dental plate ; Woodwardian Museum, Cambridge. 
Transverse striaa upon the coronal surface very prominent and 
regular in unahraded specimens. “ Upper ” dental plate regularly 
triangular, the inner margin of the adult having an average measure- 
ment of about O'Ol, tho distance from the outer apex to the inner 
margin being about 0'022. Angle between the postero-lateral and 
inner borders of tho “ lower ” dental plate very acute, and much 
elevated ; antero-latcral border also much raised, giving the plate a 
twisted appearance ; measurements of a typical specimen — postero- 
lateral margin 0'018, inner margin 0'012. 
Form. ^ Loc. Power Carboniferous Limestone : Armagh, Ireland ; 
Northumberland, England. Upper Carboniferous Limestone : Ayr- 
shire, Scotland ; Derbyshire, England. 
(i.) “ Upper ” dental plates, 
P. 2467-9. Three specimens figured by J. W. Davis, loc. cit., under 
the name of X. angustus ; Armagh. EnnisMllen Coll. 
P. 2475. Twelve similar dental plates ; Armagh. Enniskillen Coll, 
' Pal. Illinois, vol. vii. p. 175. 
0 
