TO GEOLOGY. 79 



rather deep ; marginal teeth small on the basal margin 

 larger anteriorly and posteriorly. 



Diam. .2, Length 7-20ths, Breadth 7-20ths, of an inch. 



Observations. This very curious and interesting species 

 is remarkable for its obliquity and the sculptured grooves of 

 its folds. At first sight it might be mistaken for the genus 

 Lima. The seat of the ligament is impressed, and forms a 

 small pit immediately below the point of the beak. 



The genus Pectunculus has been found in England as low 

 in the series as the Great Oolite and as high as the Crag. 

 Mr Sowerby describes five in the London Clay. In the 

 Tertiary Tables of M. Deshayes, we find twenty-seven 

 species almost equally distributed over the three periods 

 there being thirteen species in the Pliocene, eighteen in 

 the Miocene, and nineteen in the Eocene. In the Green 

 Sand of New Jersey, Dr Morton has obtained casts of 

 this genus. In the Tertiary of Maryland Mr Say has 

 observed one species. Mr Conrad has described three spe- 

 cies from the Tertiary of Claiborne and observed one other, 

 the pulvinatus of Lamarck, near York Town, Virginia. 



GENUS NUCULA. Lamarck. 



JV. Sedgewickii. Plate 3. Fig. 58. 



Description. Shell ovately elliptical, oblique, subangular 

 behind, inflated, very inequilateral, smooth ; swollen over 



