177 



least, towards the superior margin, these projections are carried out as 

 spines, to a considerable length, and in a rather curved direction, as 

 may be seen in the very slightly magnified view of them, Plate XII. 

 Fig. 8. 



T. sinuata. This is the smallest known shell of this genus. It is a 

 transverse, oblong, ovate, sirbventricose shell, with transverse winding 

 ribs, making an obtuse angle, and changing their direction both at the 

 anterior and posterior sides of the shell. Plate XII. Fig. 13. I am in- 

 debted to Mr. Clarke for specimens of this species. 



T. rudis. The form of this species is oblong, approaching to the cir- 

 cular : the cartilage slope is very indistinct : the shell, in its general 

 external appearance, very much resembles an oyster; it having a good 

 deal of that rudeness of surface which oysters in general possess. It is 

 also beset with round nodules, like those of T. davellala ; but, in this spe- 

 cies, these nodules are very irregularly disposed. Plate XII. Fig. 10, 

 shows the character of the shell ; the dotted outline, obtained by the 

 kindness of Mr. Clarke, from a fine specimen in his possession, giving 

 the form. 



The preceding five shells are found in the whetstone-pits of Devonshire, 

 are completely silicious, and possess different degrees of transparency . 



T. rugosa. Of the form of this shell I cannot speak decidedly, since a 

 fragment of it only remains. This is imbedded in a grey lime-stone, but 

 I am ignorant where it was found. The whole of its disk appears to 

 have been covered with transverse rugous ribs. Plate XII. Fig. 11. 



Shells of this genus have abounded in the Portland free stone : the 

 shells are, indeed, now decomposed and gone, but a prodigious number 

 of their lime-stone casts are found in these quarries. At Tilsbury, in 

 Wiltshire, are sometimes found calcedonic casts of this shell. One which 

 I possess, and with which I was favoured by that highly respectable gen- 

 tleman, Mr. Cunjiington, of Heytesbury, is rendered very interesting, 

 by numerous included minute bivalves being discovered in the transpa- 

 rent calcedony. The casts of these shells having been so long known as 



VOL. III. A A 



