124 TRANSACTIONS OF THE [maR. 17, 
may indicate a habit in the young shell (or in an ancestral form of 
the species) different from that of the adult; this peculiarity is 
the existence of a small boss or callus in the umbo, which, having 
a central umbilicus, appears to be homologous with the boss and 
foramen in the umbo of such shells as Acrotreta and Linnars- 
sonia.* Evidently the pedicle did not go out at this point in 
the adult shell, but the umbilicus may indicate that it did at an 
early stage of growth. According to Kutorga,+ Siphonotreta 
had a mammiform swelling around the tube in the umbo. 
The nature of the hinge connection too is very remarkable, 
and implies a mechanism similar to that of the articulate bra- 
chiopods. Compare, for instance, the dorsal valve of Orthis 
(Orthostrophia) strophomenoides, Hall,{ or O. (Dalmanella) 
testudinaria Dal.,§ with its central ridge and cardinal process, 
with that of Trematobolus; in the Orthis the sockets of the 
dorsal valve afford a rest, similar to that furnished by the elon- 
gated pit of the corresponding valve of the former genus; in the 
Orthis, however, the socket is in the inner face of the valve and 
met the tooth only of the opposite valve, whereas in our genus . 
the whole base of the dorsal is applied to the ventral. 
The genus Barroisella of the Genessee shale shows a mechan- 
isn at the hinge similar to that of Trematobolus in the bosses 
on each side of the pedicle groove of the ventral valve,|| but no 
such cardinal process is figured on the dorsal valve, as that 
which exists in the latter genus. 
In the form of the median muscular imprint of the dorsal valve, 
Trematobolus will be seen to resemble the corresponding sculp- 
ture of the dorsal valve of Obolus, as figured by Kutorga.§ In 
both genera this depression breaks up at the centre of the valve 
into three scars, of which the two outer ones arch around the 
middle one. 
Another genus which shows some interesting points of re- 
semblence to Trematobolus is Neobolus of Waagen: from the 
Salt Range in India.** This is more especially the case with 
the dorsal valve, which in the latter genus possesses a callosity 
on the median line at the hinge, similar to the cardinal process 
in our genus. The depressions on each side of this callosity 
to) 
correspond to the sears of the posterior adductor in Trematobo- 
* Op. cit. pl. iii., figs. 35, 38 and 39. 
+ Op. cit. pl. iy., figs. 22 and 23. 
tOp. cit. pl. v., fig. 26. 
Z Op. cit. pl. vB., fig. 39. ; 
| Op. cit. pl. ii., figs. 14and 15. The hinge of the dorsal valve, fig. 16 (copied from 
Meek and Worthen), evidently does not fit the cardinal line of the ventral yalve. 
qOp. Cit. p. 80, fig. 34. 
**Op. Cit. p. 84, figs, 39 and 40. 
