NUCULA. 



does not attach itself by a byssus as that of most of the 

 Arcae do. The internal ligament distinguishes Nucula 

 from all the Arcacece, 



The few species described by Lamarck^ would suggest 

 to us the observation that this is not a Genus abounding 

 in numbers, but we believe he has not mentioned one 

 third of the species, perhaps because be has not the means 

 of discriminating them. We have already mentioned the 

 localities of the known recent species. Of the fossils we 

 are acquainted with many sorts, which from our own 

 knowledge are found in the Crag, the upper marine 

 formation,'' the London Clay and Chalk Marie in Eng- 

 land, and the corresponding formations in France, par- 

 ticularly the Calcaire grossiere, at Bordeaux, Paris, and 

 in the neighbourhood of Valognes; as well as in Italy, at 

 Piacenza. The green sand also furnishes several species 

 in England. We have given representations of the fol- 

 lowing species as illustrative of the several sections of 

 the Genus we have mentioned ; 



Fig. 1. Nucula lanceolata. Lam, 



2. tellinoides, a new species lately discovered by Dr. Cheyne, and 



brought to this country from Cumana ; the following are its specific characters : 

 N. testa tenui subhyalinS,, transversim elongata, subaequilatera, antico latere 

 subangulato, postico rotundato ; striis exilioribus lineas incrementi obliqu^ 



decussantibus. 



3. Jluinatilis, Nob.; Area fiuviatilis, Schroter; N. rostrata, Lam. 



4. Pelltty Lam. 



5. rostrata, Mont, and Turton, but not of Lamarck. 



6. oblonga, Nob. j N. lanceolata, Min. Conch, t. 180; a fossil 



from the Crag. 



7. margaritacea, common on our coasts, and differing in several 



respects from any of the fossils commonly called by the same name. 



8. . pectinata, Min. Conch, t. 192, from the Chalk Marie, at 



Folkstone. 



9. — Cobboldiae, a fossil from the Crag, Min. Conch, t. 180. 



Ohs. — The first six of these have a small sinus in the 

 muscular impression of the mantle, they are all, more- 

 over, transversely elongated species, and the superior 

 edges of the shells are without crenulations. The three 

 others are crenulated at the upper edge and have no 

 sinus in the muscular impression of the mantle ; they 

 are, also, very different in general form and the hinge 

 pit is not central, and it is placed much more ob- 

 liquely. Although we think these almost sufficient rea- 

 sons for constituting two genera, as Leach did for the 

 same reasons^ the one Lembulus, the other Nucula, we 



