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From the figures given by Baier, two distinct species of this genus 

 may be made out; each of which I have been able to identify with 

 specimens in my possession. The first of these, of which I have given 

 a representation of the appearance of the outer surface, Plate XIII. 

 Fig. 12 ; and of the inner, Fig. 9, and which is by far the most common 

 species, I will distinguish as T. lata. The breadth of this shell is riot 

 much exceeded by its length, and its outer surface appears to have been 

 quite smooth, the pores seeming to show themselves only where the ori- 

 ginal surface has been removed by attrition or decomposition. 



The specimen represented Plate XIII. Fig. 10, which may be named 

 T. lamdlosa, is particularly instructive. More anxious to ascertain its spe- 

 cific characters than to preserve the specimen, however curious, I suc- 

 ceeded, with much care and time, in removing a sufficient portion of the 

 lamellae of its calcareous matrix, to enable me to discover the structure 

 on the outer surface, which I found exactly to agree with that which is 

 shown in Fig. 11, and which of course evinced that both these shells 

 were of the same species. 



Whilst attempting the removal of the matrix from another specimen 

 of this species, I was surprised at finding in the stone a spathose sub- 

 stance ramifying from the upper margin of each valve, near to the pos- 

 terior margin, and extending nearly half an inch from the shell. On 

 applying the muriatic acid to this substance, the odour soon convinced me 

 of the presence of animal matter. Hence I was led to endeavour at the 

 removal of the matrix in the present specimen, Fig. 10, with a hope of deter- 

 mining whether it possessed a similar appendage. My exertions proved 

 so successful, as to allow of the exposure of this substance proceeding 

 from both valves, as shown in the same figure. A more rigid examination 

 of the valves of the preceding species, T. lata, after this discovery, showed 

 that, on the correspondent part of the valves, a scabrous surface existed, 

 which, in all probability, had been the surface of attachment for this 

 peculiar substance. But whether this extraordinary substance should be 

 considered as extraneous, or as actually belonging to the shell, is a point 



