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/ 5 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, pi. 1, fig. 16, though evidently distinct, more nearly approaches this 

 species than Saxicava 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 lavis, of the same author, vol. ii, p. 54, t. 16, fig. 10, 

 probably belongs to a different genus ; it looks like the left valve of Montacnta bidentoAa. 



Glycimeris,* Lamk. 1801. 



Cyrtodarta. 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 wrinkled epidermis, and incapable of being withdrawn into the shell ; orifices fringed ; 

 branchice long 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./ 1S33, 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 

 Pa/iopea Norveyica. 



In Mr. Edwards's cabinet is a species from the lower division of the Older 

 Tertiaries. 



' Etym. y\vKn, dulcis, and /uepis, a species so called by Pliny from its sweet taste. 



