12 THE VOYAGE OF H.M.S. CHALLENGER. 



Marsenince, which in their youth probably agree with the other members of the family, 

 all possess a continuous, closed mantle, enveloping an internal shell, The edge is either 

 broadened out into a border, or is narrow, thick, and somewhat rounded (Marseniopsis, 

 Onchidiopsis). Besides the internal, anterior, inhalent fold and furrow, some genera 

 (Marsenina, Onchidiopsis) also possess, on the right, a similar exhalent arrangement. 

 The shell is usually calcareous, though sometimes it is almost (Marseniella), or altogether 

 (Onchidiopsis) horny. The armature of the tongue is curiously varied, and it is on this 

 ground alone that, as we have mentioned above, attempts have been made artificially to 

 break up the group. The more typical forms exhibit one lateral plate on each side of 

 the median (1 — I — 1); other genera (Marseniopsis, Marsenina, Onchidiopsis) possess, in 

 addition, two external hooks (2-1-1-1-2). But the strangest internal difference is 

 the fact, that, while the great majority of the Marseniadse are, like other Prosobranchiates, 

 unisexual, two genera, Marsenina and Onchidiopsis, are monoecious. 



Conspectus genericus. 



I pars inf. vas. def. non libera . Chelyonotus, Sw. 

 . , , , ,., t Marsenia, Leach 



pars inf. vas. def. libera . . . <^ ' 



( Marseniella, Bgh. 



nulla. I -g Is [Testa interna calcarea .... Marseniopsis, Bgb. j 



a to 



q 



)<5^3 3 — I — 3 f Testa semi-interna calcarea . . Marsenina, Gr. ) g 

 expiratoria. | Testa cornina Onchidiopsis, Bgh. j g 



The systematic position of the family cannot, in the meantime, be stated with any 

 certainty, owing to our inadequate knowledge of the anatomy of the Prosobranchiata as 

 a whole, and especially of those groups which are most probably the near relations of the 

 Marseniadse. It can at least be affirmed that they stand in close relationship with the 

 VelutinacEjE, which may, perhaps, even be afterwards denied the distinction of forming a 

 separate family. My provisional researches have shown that the latter agree with the 

 Marseniadas in the form of the nervous system, in general anatomical structure, and 

 especially in the possession of the peculiar foliated stomach. In their hermaphroditism 

 and lingual armature (2 — 1 — 1-1 — 2), the Velutinacese are still more closely linked to the 

 monoecious genera Marsenina and Onchidiopsis. Were it not for the genus Marseniopsis, 

 which is again closely bound to the Marsenice proper, one might be induced (with Gray 

 and others) to separate the monoecious genera, and to unite them with the Velutinacese in 

 one family. Meanwhile, however, the Marseniadse, as above discussed, may be regarded 

 as forming a united family. 



I. Marsenia, Leach. 



Animalia dicecia pallio ampliato, supra continuo, antice plica et semicanali inspiratoria. 

 Penis falcatus, apice dilatatus. Lamellae branchiales lateribus plicis transversalibus instructae. 



