HYPOTHYRIS. 345 



ever, great numbers of shells nearly allied to Hypotliyris 

 are found. The propriety of separating this group from 

 other Terehratula; cannot be called in question, for structure 

 of beak and foramen, peculiarity of the apophysary ap- 

 pendages, arrangement of the brachial organs of the animal, 

 and microscopic structure of the shell all combine to define 

 a natural and extensive genus. 



The account given by Dr. Carpenter of the microscopic 

 structure of the shell of our British Hypothyrh is so im- 

 portant in its bearings that we transcribe it in his own 

 words : — " This shell," he writes, " is remarkable for its 

 divisibility into thin micaceous plates, which may be split 

 into laminee of extreme tenuity. I do not know any one 

 of the Lamellibranchiate bivalves whose shell corresponds 

 with it in this respect, except Placuna and Anomia, which 

 evidently verge towards the Brachiopoda. This facility of 

 lamination characterises a large number of the fossil spe- 

 cies of the group ; especially those which correspond with 

 the one under consideration, in its peculiar characters. 

 The natural laminae thus obtained frequently afford better 

 subjects for microscopical examination than can be pro- 

 cured by making sections in the ordinary manner. When 

 these laminee are examined with a good microscope they 

 are found to present a most remarkable and characteristic 

 appearance; they are traversed by a very regular series 

 of lines, usually nearly straight, but sometimes slightly 

 curved, and running quite parallel to each other. When 

 the broken extremities of these natural laminae are exa- 

 mined, it is seen that the lines in question are produced 

 by sharp foldings of the shelly layer, which foldings are 

 parallel to each other; and this view is confirmed by 

 examination of the decalcified membrane of which only 

 one continuous stratum exists in each lamina. When the 



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