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. T. corhuloiJcs appears to have the anterior side the 

 smaller ; and although T. 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 sinus, 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 T. Conrudi. 



4. Thr.\cia ventricosa, Philippi. Tab. XXVI, fig. 5, a — c. 



TiiRACiA PUBESCENS. Phil. En. Moll. Sic, vol. i, p. 19, I. 1, fig. 10, 1836. 

 — VEXTiucos.v. Id. En. Moll. Sic, vol. ii, ]>. 17, l.'^-l-!. 



Spec. Char. Testa ovato-oblont/d, timidd, suhaquilaferali, inceqttivalvi, lenui ; antice 

 angustatd, postice iruncafd, ct ancjulatd ; valvd dcxtrd tumidiore ; umbonibus mat/nis 

 recurvaiis ; marybie veiitrali convexiuftculo. 



Shell ovatcly oblong, inequilateral, tumid, inequivalved, thin ; anterior side slightly 

 produced, posterior truncate and angulated ; right valve the more tumid ; beaks large ; 

 ventral margin slightly curved. 



Leiif/lh, If inch. Height, 1|- inch. 



LocaUly. Cor. Crag, Ramsholt, Gcdgrave. 

 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 ni)'' cabinet, which I presume to be the T. 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 particidarly so on the siphonal side, 

 differing from the preceding (?'. infiata), the shells of which are nearly smooth. Pliilippi 

 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 tlie 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. 



