374 BALANTDiE. 



ably in external appearance from the other species of the genus, and, 

 indeed, of the whole family, yet the shell in no one essential point of 

 structure materially differs from its congeners; and if we compare the 

 opercular valves with those of the three last species, we shall be struck 

 with their close, yet graduated, affinity : in P. milleporce the scuta and 

 terga tend to become calcified together, and the rostro-occludent end of 

 the scutum is bent into a different plane from the rest of the valve. In 

 P. dentatum we have an occludent ledge exactly as in the present spe- 

 cies; but in that species the adductor plate is less developed than in P. 

 monticularice ; on the other hand, in P. crenatum, the adductor plate is 

 more developed than in P. monticularice. If in P. crenatum we were 

 to remove the spur from the tergum (and it is much less developed in 

 P. dentatum ; and in P. milleporce it is entirely absent) this valve 

 would be almost identical with that of P. monticularice. Under these 

 circumstances I consider it impossible to separate the present species 

 as a distinct genus. 



Species dubice. 



The JDaracia Linnai of J. E. Gray (Annals of Philosophy, 

 new series, vol. 10, 1325), was published without descrip- 

 tion or figure. 



The Megatrema semicostata of G. B. Sowerby, junr. 

 (Conch. Manual, fig. 33, 1839), is not described, and is very 

 indifferently figured without the opercular valves, and there- 

 fore can never be recognised. 



There is an admirable figure of a Pyrgoma, without any 

 specific name, in the great c Description d'Egypte/ but from 

 the want of some details, I cannot positively recognise the 

 species ; I am inclined to believe that it is the P. dentatum 

 of this work. 



M. Chenu, in his grand Illustrations Conchyliologiques, 

 has given most beautiful figures of several species of Pyrgoma, 

 and of Creusia, but unfortunately, from the opercular valves 

 not having been figured, I find it impossible to recognise 

 them. The new species are Pyrgoma stellata (on which I 

 have appended a note under P. conjugatum), and P. spon- 

 giarum and P. corymbosa of Valenciennes. 



