80 N. ODHNER, NORTHERX AND ABCTIC INVERTEBRATES. VI. PROSÖBRANCHIA. 2 SEMIPROBÖSCIDIPERA. 



whicli is sculptured in the same manner as the embryonic Avhorls in Velutina lanigera, 

 and has a well-developed cuticula (Bergh). Marsenina, on the contrary, retains in its cal- 

 careous shell (with an extremely thin, smooth cuticula) a more primitive character. 

 Tlie close relationship of the genus Onchidiopsis to the genus Velutina is consequently 

 placed beyond all doubt, the more so, as in Velutina the mantle-border is furnished with 

 both an inspiratory and an expiratory fold. The secondary mantle-border in Velutina is 

 less dilated than in Marsenina, it is true, but the extension of the outer mantle is a char- 

 acter of little importance for judging of the affinities (cfr the variation in Marsenina). 



