TEREBRATULA. 351 



rated beak and oared valves. The distinction is very 

 probably a good one. 



We extract from Dr. Carpenter's report on the micro- 

 scopic structure of shells the following very important 

 account of the peculiar organization of the shell in our 

 native Terebratula caput sopentis, which illustrates the 

 distinctive features of the shell of Terebratula as compared 

 with that of Hypothyris. 



" When a thin portion, which has been preserved with 

 the animal in spirit, is ground down from the inner side, so 

 as to leave the outer surface unchanged, it will be seen that 

 each perforation in the shell is covered in by an oval mem- 

 branous disc, whose texture appears very firm. When a 

 thin section thus made is exposed to the action of dilute 

 acid, so as to remove from it the calcareous matter, it will 

 be seen that these discs are connected together by a layer 

 of very pellucid membrane, in which no distinct structure 

 can be made out ; this membrane, differing as it does from 

 the membranous basis of the interior layers of the shell, 

 is probably to be regarded in the light of an epidermis. 

 When a portion of the shell, not reduced in thickness, is 

 completely decalcified by immersion in dilute acid, and the 

 membranous residuum is then examined, a very remarkable 

 structure presents itself, such as is found in no shells of the 

 Lamellibranchiate Bivalves. Attached to the membranous 

 films are a series of tubular appendages, corresponding in 

 diameter to the perforations in the shell, and arranged at the 

 same distances. The free extremities of these appendages 

 are much larger than those by which they are attached to 

 the membrane, and have distinct cacal terminations, which 

 appear by the straightness of their border to have been 

 flattened against the discs that closed the orifices of the 

 perforations in the shell. Indeed in some instances these 



