BIVALVIA. 95 



pression of which may be generally seen, one on each side and above the adductors, within 

 the extended dental margin. 



Although the mantle is generally open all round, the animal is capable of contracting 

 or uniting the edges on one side, so as to form two openings, one for the incoming 

 current and the other for the outgoing, being the commencement of the true siphons. 



The known recent species of the genus are about sixty or seventy, and perhaps a similar 

 number in the fossil state ; these last are very difficult of determination, from the generally 

 slight deviations in the form of the shell, the normal condition being nearly lenticular, the 

 specific distinctions depending principally upon the sculpture of the surface or dental 

 characters ; but these teeth are very fallacious, as some are obliterated by age. 



The peculiar form of these shells are favorable to their preservation, offering, as they 

 do, a protection from mutilation, and specimens are often in high perfection. 



The genus, in the recent state, has a wide geographical extension, but the species are 

 somewhat restricted in their range ; they are principally inhabitants of warmer regions, 

 although P. glgcimeris is living in the British seas, and P. septentrionalis in those of the 

 north-west coast of America. A species found in the Eocene deposits of North America 

 is said to be identical with one of our own fossils of the same age. It is most difficult, as 

 before observed, to determine identity in shells of this genus ; but, assuming it to be as 

 so stated (which I much doubt), we may, I think, fairly place this species in the same 

 category as Terebratulina caput-serpentis, Kellia suborbicularis, and many other living 

 molluscs, whose localities at the present day are separated by apparently impassable 

 barriers. We are not able now to trace these animals, whose localities are so unconnected, 

 to what may be assumed as a common ancestry for each species thus identified. Whether 

 these apparently identical forms are descendants of ancestors belonging to the same species 

 once living together in close geographical contiguity, or whether they are forms having a 

 distinct origin, but presenting no difference by which the malacologist can separate them 

 from the typical species, we have at present not the materials to determine. 



1. Pectunculus brevirostris, /. Sowerbg. Tab. XVI, fig. 8. 



Pectunculus brevirostris. J. Sow. Min. Conch., t. 4/2, fig. 1, 1824. 



— Id. in Dixon's Geol. of Suss., p. 225, t. 14, fig. 32, 1850. 

 brkvirostrum. Morris. Catal. Brit. Fos., p. 219, 1854. 



— pulvinatus. M ant ell. Geol. of Suss., p. 2/3, 1822. 

 Pectunculus. Smith. Strata Identif., t. 11, fig. 3, 1816. 



Spec. Char. P. testa suborbiculari vel obovatd, convexo-Ienticulari vix inaquilaterali, 

 sab- symmetrica ; radiatim obsolete costellatd ; concentrice striata ; nmbonibus brevibus dc- 

 pressis ; area connexh magna, area dent alt arcuatd j dentibus paucis magnis ; marginibus 

 crenulatis. 



