1847.] SHARPE ON THE PALHOZOIC ROCKS OF N. AMERICA. 173 
of the shell, which has been completely closed up by fresh layers of 
shelly matter deposited within the shell. The foramen attains a very 
different size in different individuals, and is larger in the American 
specimens than in any which I have seen here. Probably in the 
young state the shells of this species were always attached by a liga- 
ment passing through this foramen, and at a later stage of growth 
the connection was broken and the opening closed by the deposition 
of new layers within the shell ; the period at which this change took 
place depending on local or accidental circumstances. Mr. King has 
observed a similar foramen in the closely allied species L. analoga* ; 
and M. de Verneuil has pointed it out im the American L. alter- 
natat. It also occurs in L. scabrosa of Davidson, and may some- 
times be seen in our L. euglypha. 
A similar opening will probably be found, on farther examination, 
throughout the whole genus Leptena; and it may be conjectured 
from the analogy of their structure, that all the Brachiopoda which 
have in their adult state a triangular deltidium covering up a slit in 
the hinge-area, had, when young, an open foramen at the apex of the 
dorsal valve admitting the passage of a ligament for the attachment 
of the shell. We should thus explain the cause of the break of con- 
tinuity of the hinge-area in Leptena, Chonetes, Strophomena and 
Thecidia, and we should find the deltidium in these genera analogous 
to that of Terebratula, both in its position below the foramen and in 
its use, which is to circumscribe the opening of the foramen to the 
space required for the passage of the ligament. Should this con- 
jecture be verified, we shall be able to show some mode of attachment 
in some stage of growth of nearly all the Brachiopoda, except those 
belonging to the genus Productus, and perhaps Calceola. 
(7) Orruis cARtINATA, Conrad sp.—Mr. Conrad has described 
this species in the Annual Report for 1839, p. 64, under the name of 
Strophomena carinata, as follows :—“ Shell suborbicular, with from 
sixteen to eighteen angular radiating ribs; superior valve with a 
sinus in the middle ; inferior valve angulated in the middle, slightly 
flattened on the sides; base prominent and subangulated in the 
middle ; basal margin smuous. Length three-fourths of an inch.” 
The same name has since been applied to another species by the 
same author without any explanation of the circumstance, and Mr. 
Hall has added an Orthis carinata, so that much confusion arises 
between them. 
The original species presents an unusual combination of characters 
connecting closely Orthis with Strophomena and Leptena: it has 
the open triangular foramen, which helps to define the genus Orthis, 
accompanied with broad simple plaits, such as are common in the 
species of Orthis from the Lower Silurian formation ; but with these 
characters it has the concavo-convex form of Leptena, with the 
dorsal valve convex and the ventral valve concave. The following 
may serve for a description :— 
* Annals of Natural History for July 1846, vol. xviii. p. 38. 
+ Russie, vol. ii. p. 225, and pl. 14. f. 6. 
$~ Conrad, Annual Report of the Geological Survey of New York, 1839, p. 64. 
