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MONOGRAPH OF THE ATLANTIDAE. 



I confess, that I entertain some doubt as to the specific 

 distinctness of these two species. The features by which 

 they are to be recognized, seem to be most conspicuous, 

 but they all (except those of the radula) vanish with in- 

 creasing age. 



As 1 have pointed out formerly in my monograph (p. 50), 

 and is f. i. stated by Oberwimmer, 0. keraiidreni passes in 

 its youth through a Bellerophina-siage, as it is called, and 

 it seems as if 0. rangi is only an intermediate form be- 

 tween this Bellerophina and the full-grown 0. keraiidreni. 



The specimens of Mr. Buitendijk were all young specimens 

 in different stages of development and from them I inferred 

 the following notes : 



I. Shell of Y2 Qina.: Quite chalky, wholly provided with 

 undulating spiral lines. Form much rounded, which becomes 

 yet more conspicuous by the total absence of a keel. 

 Faintly tinted in rose. (Bellerophina). 



II. Shell of 1 mm.: At the aperture a small amount 

 of cartilaginous matter is deposited; this part of the shell 

 is uncoloured and without sculpture. A very high carti- 

 laginous keel at the aperture, but only over a short distance 

 on the last whorl, and abruptly terminating. (Stage figured 

 by Macdonald, and copied by me, PI. I, fig. 5). 



III. Shell of 2 mm. : The membranous part of the shell 

 occupies the second half of the last whorl, and so does 

 the keel which indeed is intimately connected with it. 



IV. Shell of 2,5 — 3 mm. (of Souleyet): The membranous 

 part of the shell has not extended further, but the first 

 half of the last whorl, though chalky, does not show 

 the spiral lines which have retired entirely to the 

 penultimate whorl. 



All these stages are to be referred to 0. rangi. In my 

 material of 0. keraiidreni the smallest specimen measures 

 5 mm. Here the whole last whorl is cartilaginous ; the 

 keel, which is very high on the second half of this whorl. 



1) Only some transverse lines of growth are to be seen here. 



Notes from the Leyden IMuseum, Vol. XXX. 



