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, PI. XII. Fig. 8. 
T. sinuata. This is the smallest known shell of this genus. It is 
a transverse, oblong, ovate, subventricose shell, with transverse wind- 
ing 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 indebted to Mr. Clarke for specimens of this species. 
T. rudis. The form of this species is oblong, approaching to the 
circular : 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. clavellata ; but, in this species, 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 De- 
vonshire, are completely silicious, and possess different degrees ot 
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. PI. XII. big. 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 re- 
spectable gentleman, Mr. Cunnington, of Heytesbury, is rendered very 
interesting, by numerous included minute bivalves being discovered 
in the transparent calcedony. The casts of these shells having been 
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VOL. III. 
