72 PERMIAN FOSSILS. 
Australis and Leptena analoga, it may be designated an entire foramen. It is entire and 
apical in Waldheimia Australis; in Leptena analoga it is entire and supra-apical ; in 
Hypothyris obsoleta, 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 Leptena analoga, and the fissure being completely occupied by 
a prominency situated in the centre of the hinge-plate of the opposite valve. Besides, 
in Orthisina adscendens and Spirifer heteroclytus, 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, m the young state, is furnished 
with an open fissure, like that of Spirzferide ; it is afterwards closed by a deltidium, 
which, however, is furnished with a true foramen, as in Orthisina adscendens, but it is 
suspected that the foramen also became completely closed at the final period of growth. 
From what M. de Verneuil 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 Roysu, 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 Aérypa 
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 Leptena rhomboidals from the Eifel, 
with a distinct supra-apical foramen, but neither my Malvern nor Swedish specimens 
! Geology of Russia, vol. ii, p. 204, pl. xu, fig. 3. 
