OP CONCHOLOGY. 141) 



tulidce. The only portion of the internal structure of Thecidium 

 which seems clearly homologous with any part of the loop of 

 3Iegathyris, is the "bridge," so called, which, it may be sup- 

 posed, is equivalent to the crura of the last-named genus, 

 united and thickened. The loop of the Terehratididce appears 

 to me to be entirely absent, while the brachia are supported in a 

 totally different manner in the present group. 



Instead of the brachia being supported by a calcareous loop 

 of constant shape and (relative) size, as in all the genera of 

 the Terebratulid(B, they appear in Thecidium to be more or 

 less intimately attached to the mantle itself. This part of the 

 mollusk is well known to be extremely variable in its form and 

 extent. In some genera, such as Terehratella^ we find the edge 

 of the mantle varying in form during different stages of the same 

 individual, as is evident by the smooth surface, radiating ribs or 

 concentric striae which mark different parts of the surface of the 

 same shell. In some, we find that the mantle has been unsym- 

 metricai, one side being more developed than the other, while in 

 other specimens again we find more or less development of some 

 portion of the margin, while other portions are abnormally re- 

 tarded in their growth. These cases are marked by excessive 

 lateral or mesial extension of the shell, with corresponding com- 

 pression of the mesial or lateral margins. 



The mantle consists in the Brachiopoda of two laminae, the 

 one adhering to the shell and apophyses, and the other forming 

 a coat for the arms and adhering to the former. The two lobes 

 corresponding to the two valves are united only at the base of 

 the peduncle, where they form the visceral cavity. 



M. Eudes-Deslongchamps, in an extremely interesting paper 

 (Mem. Lin. Soc. de Normandie, 1864), has described the calca- 

 reous spiculae which exist in the mantle of all the genera of 

 Terebratulidce, and shown to what an extent they are produced 

 ill old individuals. Now, I believe that the mantle of Thecidium, 

 in the absence of apophyses such as serve for its support in 

 other genera, is upheld by an extraordinary deposition of such 

 spiculae, in the form of a network more or less consolidated, 

 which varies in form with the variations of the mantle, often 

 unsymmetrical on the two sides of the same individual. 



That this calcareous deposit is not homologous with the loops 

 of other genera, is shown by the fact that in those genera 

 the spiculee exist, in greater or less amount, independently of 

 the loop, which is constant in its form, while in Thecidium, 

 which has no loop, although the general form of the soft parts 

 much resembles the same parts in ^legathyris, they are deposited 

 by the variable lobes of the mantle, and serve as its only sup- 



11 



