290 MOLLUSCA FROM THE CRAG. 
There is one obtuse tooth, most prominent in the right, with a corresponding depression 
in the left valve, and the anterior muscular impression is of an ovate form, deeply 
impressed. 
I have assigned it, with some degree of doubt, as an identity with the Italian fossil, 
depending upon the very imperfect figure by Brocchi. It accords with his description, 
except that it is not “smooth,” as he describes it; but some of my specimens appear 
to have lost their granular exterior, and this may have been the case with the sub- 
Apennine fossil. 
An American shell from the Older Tertiaries, Byssomya petriculoides, Lea, ‘ Contrib. 
to Geol.,’ p. 48, pl. 1, fig. 16, though evidently distinct, more nearly approaches this 
species than Sazicava arctica, to which it has been considered to have belonged. 
Philippi has given the representation of a fossil under the name of Arcinella carinata, 
which I have considered as this species, and he refers to Mytilus carinatus, Brocchi ; 
but there is no description to assist in its determination, and my dependence is entirely 
upon his figure. Arcinella levis, of the same author, vol. 1, p. 54, t. 16, fig. 10, 
probably belongs toa different genus ; it looks like the left valve of Montacuta bidentata. 
GuyciMERis,* Lamk. 1801. 
Cyrtoparia. Daudin, 1792. Fide Gray. 
Generic Character. Shell equivalved, inequilateral, elongately oblong, thick, strong, 
flattish or compressed, gaping widely at each extremity. Hinge edentulous; ligament 
external. In the recent state covered with a thick epidermis. Two deep impressions 
by the adductor muscles, with a small sinus in the rugged and irregularly-shaped 
mantle-mark. 
Animal of the form of the shell, with the lobes of the mantle thick, and the edges, 
united, except immediately in front, where there is an opening for the passage of a 
small cylindrical foot; siphons united to their extremities, thick, and fleshy, with a 
wrinkled epidermis, and incapable of being withdrawn into the shell; orifices fringed ; 
branchize Jong and thick, two on each side. 
One recent species only of this genus has yet been noticed, and that has been 
long known. It is an inhabitant of the arctic regions, and the N. E. Coast of America, 
but not met with in our own seas. The animal of this has been examined by M. 
Audouin, and its details published in the ‘Ann. des Sc. Nat.,’? 1833, from which it 
appears to have somewhat of an isolated position. M. de Blainville placed it among 
the Naiades. In the shell and its hinge-furniture it has considerable resemblance to 
Panopea Norvegica. 
In Mr. Edwards’s cabinet is a species from the lower division of the Older 
Tertiaries. 
* Etym. yAves, dulcis, and pepss, a species so called by Pliny from its sweet taste. 
