KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 50. N:0 5. 79 



Relations. Balck (1910), with respect to the reflected lamina of the shell of O. 

 corys, which he supposes to be homologous to the posterior wall of the mouth space 

 in Marsenina, discusses the relations of the genus Onchidiopsis and includes it in the 

 subfam. Velutininae, where he considers it most closely allied to Marsenina, »the new 

 species . . . partially bridging the gap and indicating in a most interesting way the pre- 

 cise manner in which the still whorled shell of Marsenina has degenerated, in this series, 

 to a mere scale of the other species of Onchidiopsis» (P. 478). 



The anatomical structure of Marsenina seemed, as far as was then known, to sus- 

 tain this assumption. But in the present work I have shown that in the genital appar- 

 atus, Marsenina represents a stage intermediate between Lamellaria and Velutina, while 

 Onchidiopsis, in the structure of the genital organs, approaches to Velutina. Further, 



Fig. 5. Tlie Genital Apparatus of Onchidiopsis groenlandica Beegh. 



the radula of Marsenina, judging from the shape of the teeth (simply denticulated), has 

 probably been subject to a reduction (from a type represented by Marseniopsis; cfr Bergh 

 1887), and is, by that circumstance, widely separated from that of Oncliidiopsis which 

 is on the contrary closely allied to the Velutina type. 



As regards the shell there is no doubt that the relationship of Onchidiopsis 

 to Velutina is more probable than to Marsenina. In the general shape the shells of 

 Velutina and Marsenina are similar (cfr Pl. 1), and from this point of view, the genus 

 Onchidiopsis might as well be allied to Velutina as to Marsenina. But in the last genus 

 the structure is calcareous, while in Velutina the shell is subject to a reduction of the cal- 

 careous layer resulting in a cartilagineous or corneous structure (V. plicatilis). From 

 such a form the shell of Onchidiopsis must have developed by extreme dilatation of the 

 aperture and reduction of the spire. In fact the embryo of Onchidiopsis has a spiral shell. 



