192 
SELACHIT. 
. 
from the outer apex to the inner margin about 0-035. 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, Sf Loc. Lower Carboniferous Limestone : Bristol. 
20575, 38198, 41162. Thirteen large dental plates. 
Purchased , 1846, 1861, 1868. 
39169. Abraded large dental plate, with a fragment, either of 
cartilage or of the root of a smaller tooth, attached 
posteriorly. BowerbanTc Coll. 
P. 1403, Twelve large dental plates, some showing indications of 
&--k. wear in the middle of the crown. Egerion CoV. 
P. 2492. Four large examples and five of the small variety. 
EnnisJcilJen Coll. 
20575 a, 20828, 41169. Six small dental plates. 
Purchased , 1846, 1847, 1868. 
No other undoubted species of Tomoclus is yet known. The 
dental plates from the Belgian Lower Carboniferous Limestone, 
described by L. G. de Koninck under the names of Tomoclus craigi 1 
and T. laciniatus 2 , may be referred respectively to Psephodus and 
Pletiroplax. Another supposed species from the Burlington Lime¬ 
stone (Lower Carboniferous) of Iowa, U.S.A., is also uncertain, 
namely Tomodus ? hmitcivis, St. John&W orthen, Pal. Illinois, ol. vii. 
(1883), p. 173, pi. xiii. fig. 12. ^ 
Genus' XYSTRODUS, Agassiz. 
[Morris & Boberts {ex Agassiz, MS.), Quart. Journ. Geol. 
Soc. vol. xviii. 1862, p. 101k] 
Dental plates small, thin, delicate, triangular in outline, gently' 
arched, with the outer apex more or less inrolled; coronal surface 
exhibiting the punctations 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. 
1 Faune Calc. Carbf. Belg. pt. i. 1878, p. 61, pi. iv. fig. 8, pi. vi. figs. 18, 19. 
■2 Op. cit. pt. i. p. 61, pi. vi. fig. 20. 
3 The name only is here mentioned and applied to the species previously 
described and figured by M‘Coy under the name of Cochliodus striatus. The 
type species being thus already well-defined in 1862, the generic name may be 
regarded as dating from that year. 
