186 
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 not 
much exceeded by its length, and its outer surface appears to have 
been quite smooth, the pores seeming to show themselves only where 
the original surface has been removed by attrition or decomposition. 
The specimen represented Plate XIII. Fig. 10, which may be named 
T. lamellosa, is particularly instructive. More anxious to ascertain 
its specific characters than to preserve the specimen, however curious, 
I succeeded, with much care and time, in removing a sufficient por- 
tion 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 spatliose 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 determining whether it possessed a similar appendage. My 
exertions proved so successful, as to allow of the exposure of this sub- 
stance proceeding from both valves, as shewn 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 sur- 
face of attachment for this peculiar substance. But whether this ex- 
traordinarysubstance should be considered as extraneous, or as actually 
