METAZOA MOLLUSCA. . 227 



The series of Cypraea cervus Linn. (No. 489 a-c) is 

 extremely instructive. The young shell (a) is a spiral 

 in which the number of whorls can be easily counted. 

 A still earlier stage, of which there is no specimen, is a 

 simple, thin, snail-like spiral (Adams). The shape of (a) 

 is similar to Oliva, Cymbium, and the like. The aperture 

 is also long with a thin, sharp, untoothed, and unorna- 

 mented margin, and a slight notch at the anterior end. 

 In (b) the shape has changed, the margin of the aperture 

 has thickened and turned inward, while both sides of the" 

 opening are toothed and both ends notched. In (c) only 

 vestiges of the spiral remain, which no one would recog- 

 nize who had not seen the younger stages ; thus the shell 

 has taken on a character that separates it from those pre- 

 viously described and which places it with the more spe- 

 cialized forms. If a vertical section of the shell is made, 

 the whorls are seen inside (No. 490, C. exanthema Linn.) 

 concealed by the last body whorl. No. 489 c is of re- 

 markable size and is lighter in weight than some of 

 smaller size. 



It is thus seen that the development of Cypraea is an 

 epitome of the life history of those Gastropods which 

 have a simple, snail-like spiral in adult life, and of those 

 with the modified spiral of Oliva and Cymbium. 



A still more specialized form is Cypraea mauritiana 

 Linn. Its development is shown in the series No. 491. 

 The first six specimens would hardly be called the young 

 of the last six, so unlike are they in shape and coloring. 

 In the youngest specimen in the series, the spire with its 

 small whorls and large body whorl is seen. The shell 

 is now strikingly like Oliva. The long aperture has a 

 sharp margin which as yet has not turned inward. In 

 the older stages the shape changes ; the early spiral is 

 concealed by the enamel which the mantle has laid on. 

 There are two broad, thick, flat, toothed lips with a nar- 

 row aperture and a canal at either end. The two largest 

 specimens have grown high vertically, and this peculiar 



