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 Mucula 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. Nucuna ampia, Ldwards, MS. Tab. XVIII, fig. 5, a, 4, var. fig. 6, a, 6. 
Spec. Char. NV. testa transversd, ovato-subtrigonuld vel nuciformi, ampla, tumidiusculd, 
crassa, levigata ; pedi-regione latiore ; ano-regione paulo attenuata, rotundatad ; margine 
dorsali convextusculd ; margine ventrali plis convexd ; lunuld elongato-lanceolatdé ; dentibus 
ad apicem gradatim minutis ; marginibus crenulatis. 
Shell transverse, ovately 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, 3 an inch. 
Locality. Barton (Hdwards). 
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 JV. /uzulata, 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. 6, a, 6, 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. 
