72 PERMIAN FOSSILS, 



Australis and Leptana analoga, it may be designated an entire foramen. It is entire and 

 apical in Waldheimia Australis ; in Leptana analoga it is entire and supra-apical ; in 

 Hypothyris ohsoleta, entire and sub-apical ; and in Cleiothyris pectinifera and Terebratula 

 caput-serpentis, emarginate and apical. 



The distinction between a foramen and a fissure is indispensable, as the former 

 appears to have served in all cases as an opening for a pedicle, by which the 

 Palliobranchs possessing it were attached to foreign bodies ; but the latter structure, 

 even when open, did not always occupy this office, as proved by the presence of a 

 supra-apical foramen in Leptcena analoga, and the fissure being completely occupied by 

 a prominency situated in the centre of the hinge-plate of the opposite valve. Besides, 

 in OrtJdsina adscendens and Spirifer Jieterodytiis, which have the fissure closed by a 

 deltidium, this last structure is furnished with a foramen. It must be admitted, 

 however, that an open fissure, in the absence of a foramen, must have served as a 

 passage for the pedicle ; and it is evident, when a fissure became filled up by the 

 deltidium, and conditions rendered it still necessary for the shell to remain attached, 

 that a provision was made for the contingency by the addition of a foramen in the 

 most suitable place. 



Increase of age evidently induced, in some instances, a modification of the struc- 

 tures under consideration : thus Strigocephalus Burtini, in the young state, is furnished 

 with an open fissiire, like that of Spiriferidce ; it is afterwards closed by a deltidium, 

 which, however, is furnished with a true foramen, as in OrtUsina adscendens, but it is 

 suspected that the foramen also became completely closed at the final period of growth. 

 From what M. deVerneuil states respecting the last shell,^ it appears to be subject to a 

 similar modification ; but in this instance the modification has the appearance of being 

 merely varietal, and not apparently specific, as in Strigocephalus. 



The foregoing will tend to clear up certain discrepant statements which have been 

 published on the structure last noticed — one writer describing a shell as perforated, 

 and another stating it to be imperforate : thus Spirifer heteroclytus is in this predica- 

 ment. A specimen of Cleiothyris Boysii, at present before me, exhibits a distinctly- 

 defined, incomplete, apical foramen ; yet there are several who state that it is 

 imperforate : it is the same with some other species of this genus. The mutability of 

 nearly all the hinge-characters is curiously displayed in Rhenish specimens of Atrypa 

 reticularis : in some there is no appearance of a foramen ; in others it is distinctly visible : 

 again, in one the foramen is entire and apical ; in a second, sub-apical and entire ; in 

 a third, apical and incomplete ; and in a fourth, sub-apical and incomplete : some 

 specimens are even provided with a distinct area. So liable is the foramen to become 

 closed, that it seems unsafe to reject a specimen from any given perforated species, 

 because it is imperforate. I have specimens of Leptana rhomhoidalis from the Eifel, 

 with a distinct supra-apical foramen, but neither my Malvern nor Swedish specimens 



^ Geology of Russia, vol. ii, p. 204, pi. xii, fig. 3. 



