COCHLIODOUTTID^;. 
193 
As already remarked by St. John and Worthen the known dental 
plates of each species are of two forms—the one, presumably per¬ 
taining to the upper jaw, but little inrolled and very narrow antero- 
posteriorly, the other considerably inrolled and of the reverse 
proportions. 
So far as known, Xystrodus may be regarded as closely allied to 
Tomodus, the dental plates only differing from the small plates of 
the latter genus in their less curvature and in the peculiar arrange¬ 
ment of the minute coronal punctations. 
Xystrodus strlatns (M‘Coy). 
1843. Cochliodus striatus, L. Agassiz, Poiss. Foss. vol. iii. p. 174 (name 
only). 
1855. Cochliodus striatus, F. M‘Coy, Brit. Pakeoz. Foss. p. 624, pi. 3 1 . 
fig. 27. 
1862. Xystrodus striatus, Morris & Boberts, Quart. Journ. Geol. Soc. 
vol. xviii. p. 101 (name only). 
1862. Xystrodus anyustus, Morris & Boberts, tom. cit. p. 101 (name 
only). 
1883. Xystrodus striatus, J. W. Davis, Trans. Boy. Dublin Soc. [2] 
vol. i. p. 448, pi. liv. figs. 7-10. 
1883. Xystrodus anyustus, J. W. Davis, tom. cit. p. 449, pi. lv. 
figs. 19-21. 
1883. Xystrodus striatus , St. John & Worthen, Pal. Illinois, vol. vii. 
p. 183. 
Type. “ Upper ” dental plate ; Woodwardian Museum, Cambridge. 
Transverse striae upon the coronal surface very prominent and 
regular in unabraded specimens. “ Upper ” dental plate regularly 
triangular, the inner margin of the adult having an average measure¬ 
ment of about 0*01, the distance from the outer apex to the inner 
margin being about 0-022. Angle between the postero-lateral and 
inner borders of the “lower” dental plate.very acute, and much 
elevated ; antero-lateral 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. Sf Log. Lower 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. anyustus ; Armagh. Enniskillen Coll. 
P. 2475. Twelve similar dental plates ; Armagh. Enniskillen Coll . 
1 Pal. Illinois, vol. vii. p. 175. 
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