191 
It resembles nob., but in that species the cardinal 
teeth are direct, and a remarkable feature is exhibited by this 
shell in its anterior rectilinear declivity, terminating below the 
anterior middle in a kind of rostrum or prominent angle, which 
may be compared to that of nasutus, N., excepting that it is nearly 
on a line with the base. I have not seen any variety of either of 
those species that could be mistaken for this. The beaks and um- 
bones in my specimens are very much eroded, exposing a wax 
colored surface. PI. 35. 
Arca. — Shell transverse, more or less elongated, and oblique ] 
anteriorly subrhomboidal, inequilateral, subequivalve ; summits re- 
mote hinge rectilinear or slightly curved and furnished with a 
series of numerous, vertical inserting teeth, which decrease in size 
from the extremities to the centre ; two muscular impressions, re- 
mote, connected by a simple parallel line; ligament broad, extend- 
ing both before and behind the summits, and expanding over the 
rhomboidal surface beneath the summits ; animal with a byssus. 
Ohs. Linne included in his genus Arca, not only the species 
of which it is at present constituted, but those also which have 
been since distinguished by the names of Pectunculus 20 idi Nucula. 
Gmelin added to it his A. cucullus, afterwards separated by La- 
marck under the generic name of Cuculloea. 
As respects Nucula it appears that the relation to Arca is rather 
one of analogy than of affinity, and it is possible that Turton may 
be right in separating it entirely from the Arcacesc, with which 
its numerous teeth bear some resemblance ; but the propriety of 
placing it in the family of Mactracesc, only by the character of the 
internal ligament, seems to be questionable. 
Cuculloea corresponds with the present genus in its general ap- 
pearance, and we know of no other difference than that exhibited 
by the teeth, which are less regular, and those at each extremity 
of the series are transversely elongated. 
Pectunculus is a closely related genus, differing chiefly by the 
arquated series of teeth ; the general orbicular form, and by the 
valves closing completely so as to prevent the passage of a byssus. 
Ferussac in his Tabl. Syst., places Trigonia in this family; but 
the teeth are dissimilar, and the animal is unknown. 
The Arcae are marine shells, and, as Lamarck observes, some of 
the species have one of the valves extending beyond the other on 
