191 



It resembles ^Mr/)MreMS, nob., but in that species tbe 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 nasutiis, 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 j 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. 



Obs. 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 and Nucula. 

 Gmelin added to it his A. cucuUus, afterwards separated by La- 

 marck under the generic name of CucuUoea. 



As respects Niccula 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 Arcaccx, with which 

 its numerous teeth bear some resemblance ; but the propriety of 

 placing it in the family of Mactra^ese, only by the character of the 

 internal ligament, seems to be questionable. 



CucuUoea 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 Arcse, are marine shells, and, as Lamarck observes, some of 

 the species have one of the valves extending beyond the other on 



