192 
sKr.Acnrr. 
from t,ho outer apex to the inner margin about 0'0:}5. Small plate 
relatively narrower, the lateral margins of the crown more or less 
raised, with a median hollow ; maximum distance from the outer 
apex to the inner margin about 0-018. 
Form. ^ Lor. Lower Carboniferous Limestone : Bristol, 
80575, 36198, 41162. Thirteen largo dental plates. 
Purcham/, 1840, 1801, 1808. 
39169. Abraded largo dental plate, with a fragment, either of 
cartilage or of the root of a smaller tooth, attached 
posteriorly. Bovierbank Coll. 
P. 1403. Twelve large dental plates, some showing indications of 
wear in the middle of the crown. Egerlon Coll. 
P. 2492. Four largo examples and five of the small variety. 
Enniskillen Coll. 
20575 a, 20828, 41169. Six small dental plates. 
Purchased, 1840, 1847, 1868. 
No other undoubted species of Tomodus is yet known. The 
dental ))lates from the Belgian Lower Carboniferous Limestone, 
described bj' L. G. de Koninck under the names of 'Pomodus erai(/i ’ 
and T. laciniatus m'dy be referred respectively to Psephodus and 
Pleuroplax. Another supposed species from the Burlington Lime- 
stone (Lower Carboniferous) of Iowa, U.S.A., is also uncertain, 
namely Tomodrts‘1 limitaris, lit. Jobn&Worthcn, Pal. Illinois, vol. vii. 
(1883), p. 173, pi. xiii. fig. 12. 
Genus XYSTRODXJS, Agassiz. 
[Morris & Roberts (e.v Agassiz, MS.), Quart. Journ. Geol. 
Soe. vol. xviii. 1862, p. 101’.] 
Dental plates small, thin, delicate, triangular in outline, gently 
arched, with the outer apex more or less inrollcd ; coronal surface 
exhibiting the puuctations closely arranged in transverse parallel 
rows, generally producing a definite striated appearance. Antero- 
lateral portion of the crown much raised, and thicker than the pos- 
tero-lateral, which is also slightly' upturned at the margin. 
' Faime Calc. C'arbf. Belg. pt. i. 1878, p. 61, pi. iv. fig. 8, pi. vi. figs. 18, 19. 
* Op. cit. pt. i. p. 61, pi. vi. fig. 20. 
■'* The name only is here rnentionecl and applied to the species previously 
described and figured by M'Ooy under the name of CocMwdus sfiiafus. The 
type species being thus already well-defined in 1862, the generic name may be 
regarded as dating from that year. 
