402 THE MEDALS OF CREATION, Cuap. XI. 
and oolitic formations; and very recently four or five species 
have been discovered in the Silurian strata of Ireland.* 
These shells are chiefly characterized by their hinge (see 
Lign. 129, fig. 1a.), and by the fibrous structure of their 
constituent substance, which closely resembles that of the 
recent Pinna; and under the microscope is found, like 
that shell, to consist of prismatic cells, filled with carbonate 
of lime.+ The species vary in size from an inch to three or 
four feet in diameter. The shell, in consequence of the 
vertical arrangement of the fibres, readily breaks to pieces, 
and it is often extremely difficult to extricate a specimen 
with the hinge and beaks tolerably entire. That they were 
equally brittle when recent is evident from the numerous 
fragments diffused through the chalk and flint, and occa- 
sionally imbedded in pyrites.§ The form of the hinge is 
shown in Lign. 129, fig. 1: in the lower specimen two 
valves of the same individual are seen displaced, one lying 
over the other. The usual chalk species are figured ss. 
South D. pl. xxvii. and in Min. Conch. 
In the Galt, or Folkstone marl, two small species of this 
genus are to be found in every locality I have visited. They 
were first figured and described by Mr. Parkinson, under 
the name of Jnoceramus sulcatus, and I. concentricus (Wond. 
p- 330, fig. 1 and 3). In most examples the shell is in 
* The term Inoceramus is restricted by the French geologists to 
the beaked and laminated species of the Galt; and the chalk Inoce- 
rami are arranged under the name Catillus. 
+ Perna and all the Aviculide have the same structure. Inoce- 
ramus scarcely differs from Perna. 
~ Dr. Carpenter on the Microscopical Structure of Shells. To 
detect this structure, the shell should be immersed in diluted hydro- 
chloric acid, and when partially dissolved, the cells will be apparent. 
§ It was many years before I succeeded in obtaining a specimen 
with the hinge perfect; and M. Brongniart, unable to obtain one 
from the chalk of France, gave the figure of this genus from my 
Foss. South D. pl. xxvii. in the Géog. Min. Env. de Paris. 
