PERNA. MONOMYARIA. 85 
of part of the ligament; muscular impressions almost obso- 
lete, of an oblong form, and situate near the anterior mar- 
gin of the pearlaceous substance. 
Crenatula ventricosa. Plate IX. fig. 18. 
‘ The greater portion of the shell of the Crenatule consists of a foliaceous 
substance, composed of perpendicular fibres, like the Perne, Ostrza, &c. They 
are distinguished from the Perne, by being destitute of a passage for a byssus ; 
and the hinge of the latter genus has a series of straight ligamentary grooves 
placed across it. The Crenatule are obliquely elongated shells, while the 
. Perne are rather transverse. 
They occur but rarely in a fossil condition, and chiefly in the Oolitic group 
of rocks. 
Genus XXXII. — CATILLUS. — Brongniart. 
Generic Character. — Shell thick, inequivalve, subequi- 
lateral ; triangular, deep, with incurved umbones;_ hinge 
consisting of a series of transverse grooves. 
Catillus Lamarchit. Plate VIII. fig. 17. C. sulcatus. 
Plate VII. fig. 20. The first is found in the chalk of Sus- 
sex, and the latter in the chalk marl. 
Section 1.— Beaks shert ; valves nearly equal. 
Section 2, — Beaks elongated ; valves unequal. 
This genus consists entirely of fossil species, which are chiefly found in the 
chalk ; a few shells have, however, been found agreeing with the Catille in 
the Todmorden shales, which I have referred to this genus. 
Genus XXXIV. — PERNA. — Bruguiere. 
Generic Character. — Shell subequivalve, flattened, and 
somewhat irregular, a little distorted, thickish, and exter- 
nally lamellar ; the laminz composed of minute perpen- 
dicular fibres ; beaks small, nearly equal, and situate at the 
posterior extremity of the hinge margin; hinge linear, 
marginal, with numerous transverse, parallel, opposite 
grooves, which, together with flattened ridges between 
them, are destined for the reception of the ligament ; the 
anterior extremity of the hinge is narrower than its poste- 
rior termination ; situate immediately under the extremity 
of the hinge margin is a posterior sinus, for the passage of 
the byssus ; with a parietal callosity, which is more distinct 
in the right hand valve than in the opposite ; the interior 
pearlaceous substance of the shell is spread out almost in 
the same form as the exterior, fibrous, and more extended 
portion ; one distinct, somewhat oblique and irregular mus- 
- cular impression, and a series of small dots, are placed at 
