BRACHIOPODA. 



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Terebratula. The shells of the last named genus are 

 finely perforated, the apertures being filled up in those 

 living, by membranous or cellulose closed sacs. 



Dr. Carpenter thus speaks of the curious formation of 

 the shelly matter in this class: — i( One of the most in- 

 teresting points in the structure of Terebratula, or at least 

 in certain species of it, is the existence of a large number 

 of tubular perforations, passing directly from one surface 

 of the shell to the other, and terminating by an orifice at 

 each. The size of these perforations is sufficiently great 

 to enable them to be detected with a hand magnifier, as 

 minute punctations on the surface ; and in this manner I 

 have observed them in all the recent species of Terebratula 

 (about fourteen) which have come under my notice, except 

 in T. psittacea, which departs so widely from the general 

 type in the incompleteness of the passage for the ligament 

 that (as Mr. Stutchbury has suggested to me) it is pro- 

 bably to be considered as the recent type of Spirifer. Of 

 the very numerous fossil species of Terebratula I have yet 

 examined but a small proportion ; yet the curious result 

 has uniformly presented itself, that the perforations have 

 invariably been found in the non-plicated, or moderately 

 plicated species, whilst they have been absent in those 

 which differ from the recent species, in being much more 

 deeply plicated." 



The Brachiopoda are pre-eminently creatures which 

 have belonged to past times, hence the great number of 

 fossil specimens, and the multiplicity of their forms, whilst 

 comparatively, the number of living Brachiopoda is small, 

 and they offer few distinctive characters. Mantell says 

 there are five hundred fossil species known in the British 

 strata alone. The living species are found in all seas, 

 even on the icy coast of Spitsbergen, and usually prefer 

 the deep seas. 



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