262 MOLLUSCA FROM THE CRAG. 
From the want of a good series of very perfect specimens, it is difficult to say 
whether. this be really distinct. 7. corbuloides appears to have the anterior side the 
smaller; and although 7’ Conradi comes very near to our Crag fossil, it differs also in 
the same character, and has a rather more prominent umbo. A difference also is shown 
in the smus, which is not only deeper, as it would naturally be from the difference in 
the proportion of the sides, but it appears somewhat more angular in the recent 
American shell. I am, however, inclined to believe, that when a better comparison 
can be instituted, it may be found not to differ specifically from 7. Conradi. 
4, THRACIA VENTRICOSA, Philippi. Tab. XXVI, fig. 5, a—e. 
THRACIA PUBESCENS. Phil. En. Moll. Sic., vol. i, p- 19, t. 1, fig. 10, 1836. 
— veNTRICOSA. Jd. En. Moll. Sic., vol. ii, p. 17, 1844. 
Spec. Char. Testa ovato-oblongd, tumidd, subequilaterali, inequivalvi, tenui ; anticé 
angustatd, postice truncatd, et angulala ; valvéd dextrd tumidiore; umbonibus magnis 
recurvatis ; margine ventrali convewiusculo. 
Shell ovately oblong, inequilateral, tumid, inequivalved, thin; anterior side slightly 
produced, posterior truncate and angulated; right valve the more tumid; beaks large ; 
ventral margin slightly curved. 
Length, 1% inch. Hetght, 13 inch. 
Locality. Cor. Crag, Ramsholt, Gedgrave. 
Fossil, Sicily. 
Two or three specimens only have come into my possession. They are assigned to 
the Sicilian species with a slight degree of doubt, differing somewhat from a 
Sicilian fossil in my cabinet, which I presume to be the 7. ventricosa. It is rare to find 
the fossils belonging to this genus in a perfect condition, their extreme thinness being 
insufficient to preserve them in their natural form, and their characters in consequence 
are difficult to determine. 
Our fossil is finely granulated all over, more particularly so on the siphonal side, 
differmg from the preceding (7'. zvflata), the shells of which are nearly smooth. Philippi 
represents his shell as equivalved; but in my Sicilian fossil there is an evident 
inequality in the valves, and the proportions of the two sides do not exactly cor- 
respond. 
There is a linear depression for the ligamental portion of the hinge, and the support 
for the cartilage is very narrow, giving it almost the appearance of having an entirely 
external ligament. 
T. convexa is said by Messrs. Forbes and Hanley to be found fossil in the Newer 
Tertiaries or Pleistocene Clays of Belfast, as also in the Clyde Beds. 
