108 EOCENE MOLLUSCA. 



by Messrs. Adams. I have not seen this kind of sculpture upon any Eocene fossils. In 

 this genus it is often difficult to determine, in descriptions, which part is intended for the 

 " anterior," as that term is applied sometimes to the shorter, at others to the longer 

 division of the shell. 



It has been generally supposed that the species of Nucula are well defined and easily 

 determined, but I am sorry to say, they have not appeared so to me. The Eocene species 

 have given me more trouble in their assignment than those of almost any other genus, 

 and the result is not at all satisfactory to myself; it will be fortunate for me if I be the 

 only one of that opinion. The Eocene shells of this genus found in England, and here 

 illustrated, present a great variety of forms, most of which I have considered as entitled 

 to specific distinction ; it is however possible, that with a larger amount of materials, 

 some of these lines of division might disappear. 



1. Nucula ampla, Edwards, MS. Tab. XVIII, fig. 5, a, b, var. fig. 6, a, b. 



Spec. Char. N. festd transversa, ovafo-subfrigonuld vel nuciformi, ampla, tumidiusculd, 

 crassd, laevigata; pedi-regione latiore ; ano-regione paulo attenuatd, rotundatd ; margine 

 dorsali convexiusculd ; margine ventr all plus convexd ; lunula elongato-lanceolatd ; dentibus 

 ad apicem gradatim mimttis ; marginibus crenulatis. 



Shell transverse, ovatelv trigonular or nut-shaped, broad, somewhat tumid, thick, 

 smooth ; pedal region the broader ; anal region slightly projecting and rounded ; dorsal 

 margin slightly convex, ventral margin more rounded ; lunule elongated ; denticles 

 diminishing towards the apex ; margins crenulated. 



Length, \ an inch. 



Locality. Barton {Edwards). 



This species, I believe, is not very rare. Its peculiar or specific distinction is the 

 roundedly ovate form, which appears to be more so than in any other species I have seen. 

 The anal region is much rounded, and the dental area on that side short, by which is 

 given a greater convexity to the ventral margin. It has an elongated indistinct lunule or 

 dorsal depression, with a slightly elevated corselet surrounded by a depression; it bears 

 some resemblance to N. lunulata, Nyst, but it appears to differ from that species in having 

 both dorsal and ventral margins more curved, and it has not so distinct and prominent a 

 corselet as that species. The interior is sometimes much thickened, and the adductor 

 marks are deep ; there is also an elongated impression of the pedal muscle near the oral 

 adductor, beneath the dental margin, and generally in thickened specimens an upright 

 visceral (?) impression. Fig. Q,a, b, in same plate, represents a specimen in Mr. Edwards's 

 cabinet with the MS. name of contigua ; in this there is a slight difference in the contour, 

 and in the anal region, but I think it is scarcely entitled to specific distinction. I have 

 therefore considered it only as a variety of the above species. 



