KEPOKT ON THE MARSENIADSE. 11 



state that they eat out round holes in composite Ascidians (Leptoclinum maculosum, 

 Polyclinum succineum), and there deposit their ova. They shut the cavities with special 

 lids which exhibit concentric rings, said to be produced by the rotatory movements of the 

 animals. The Onchidiopsides seem to form cavities with somewhat similar lids in a 

 Halisarcid (and probably also elsewhere). The brood-cavities contain, as Giard (Marsenia) 

 and Bergh {Onchidiopsis) have observed, a number of nutritive ova which supply the 

 developing embryos with food. Giard 1 has traced the general outline of the develop- 

 mental history ; according to him, two successive provisional shells are formed, the first 

 nautiloid, the second simpler, exhibiting a greater resemblance to that of a Carinaria. 

 Long before, however, Krohn 2 had noted that the larval shell of these animals does not 

 develop into that of the adult, but that within the nautiloid shell, which is provided with 

 several keels, a new shell is formed with a much greater resemblance to that of the adult 

 Marsenia. The animal withdraws from the primary shell (which then disappears), and is 

 finally covered only by the rudiment of the new persistent shell. Similar observations 

 have been made by Macdonald 3 on South Sea forms, as the result of which it would seem 

 that the Jasonillce of Macdonald, the Brownies of d'Orbigny, and the Calcarellce of 

 Souleyet are probably simply larval forms of Marsenim or other Marseniadse. I have 

 also observed a larva with a somewhat similar, but much ribbed, primary shell, and this 

 is probably to be referred to an Arctic Onchidiopsis.* 



The Marseniadse are found in all seas ; particularly developed types mostly occur in 

 tropical seas (Chelyonotus) and the Arctic Ocean {Onchidiopsis). As to their distribution 

 in time, they seem [Marsenia) to have appeared late in the tertiary period. 



Although the Marseniadse form, both in habit and structure, a tolerably well-defined 

 group, they differ among themselves not a little in both these respects, but especially in 

 the armature of the tongue ; and it is because of this that the different members of the 

 family have been repeatedly severed from one another. 5 With the exception of the 



1 Giard, Sur l'embryologie du Lamellaria perspieua, Comptes Rendus, 22 Mar. 1875, pp. 736, 738, 739. 



2 A. Krohn, Ub. einen neuen mit Wimpersegeln versehenen Gasteropoden, Archiv f. Naturgeseli., Jahrg. 



xix., Bd. i., 1853, pp. 223-226, taf. xi. figs. x. i.-ii. 

 A. Krohn, Nachtr. zu dem Aufs. ub. die Echinospira diaphana, Archiv f. Naturgeseli., Jahrg. xxv., Bd. i., 



1855, pp. 1-5, taf. i. figs. 1, 2. 

 A. Krohn, Eernere Nachtr. zu dem Aufsatze lib. dielEchinospira, nebst Bemerk. lib. eiue ihr verwandte 



Larve, Archiv f. Naturgeseli., Jahrg. xxiii., Bd. i., 1857, p. 252, taf. xii. 



3 Macdonald, Further Observations on the Metamorphosis of Gasteropoda, Trans. Linn. Soe. Lond., 

 1860, vol. xxiii. pt. 1, p. 70. 



4 A similarly caducous larval shell appears to have been hitherto observed only in the CymbulicB (by 

 Krohn), where the gelatinous shell of the adult seems to be an altogether independent formation. 



5 Troschel (Das Gebiss d. Sclmecken, 1856-1863, vol. i. pp. 167, 185) placed the Marsenia} and the 

 Chelyonoti next the Natieaeem ; the Onchidiopsides and the Marseninm next the Velutinacece, as had been 

 already proposed by Gray (Guide, 1857, vol. i. p. 46), who had incorporated the former in the Velutinidae. 



Gray wished (Guide, 1857, vol. i. pp. 27, 28) to divide the family into three generic groups, the Coriocellae, 

 the Lamellarice (with Lamellaria tentaculata as type), and the Ermew (with Ermea perspieua as type) ; an 

 unreasonable proceeding in every respect, as to names, characterisation, and grouping. 



