Dr. 0. A. L. Morch on the Operculum and its Mantle. 1 19 



Duthiers*) often produces posteriorly a shell {operculum) analo- 

 gous to the shell of the true mantle ; laterally it can be deve- 

 loped into fins, as in Aplysia and Gymnosomata (Pneumoder/non, 

 &c), analogous to the pallial fins of the Cephalopoda; it can 

 form a fimbriated or undulated edge, as in Haliotis, Trochidee, 

 Elysiidce, and Philinef; it can form a siphon, as in Cephalopoda 

 or in the American Ampullarice, analogous to the mantle-siphon 

 of Buccinidce ; perhaps it forms anteriorly the tentacula, ommato- 

 phores J, and intertentacular lobes of Trochidee, corresponding to 

 the mantle-edge of Peciines, Solenes, Galeomma, &c. The epi- 

 podium is attached to the foot ; but it is not quite clear that it 

 is a part of it. One author considers the foot homologous with 

 the adductor muscle of the Acephala ; but it must be remembered 

 that the foot of the Acephala is homologous with that of the 

 Gasteropoda. Dr. Gray regards the muscles which connect the 

 columella and the operculum as homologous with the adductor 

 muscles of bivalves. 



Prof. Loven considers the operculum homologous with the 

 byssus ; but, as this organ is found in several univalves, even in 

 those with an operculum, this opinion cannot be adopted, as was 

 pointed out by Prof. Macdonald. It must also be remembered that 

 it is not known how the byssus of univalves is formed. Swain- 

 son (Treatise, p. 186, f. 29) represents Cyclostoma suspensum, 

 Gould, and A. Adams (Voyage of Samarang, p. 44, 1. 13. f. 3), 

 Cerithidea obtusa, Lam., as attached to a branch by a byssus 

 during the aestivation. According to Macdonald, Planaxis, and 

 to Gray, Rissoa parva, spin a byssus. It is possible, from its re- 

 semblance to that of Mytilus, that the deep posterior groove in 

 the footsole of Cerithiopsis tubercularis (Forb. & Hani. Brit. 

 Moll.) produces the byssus. The nature of the slimy thread of 

 Litiopa, too, is very doubtful ; perhaps it only corresponds to the 

 thread of Limax filans, Hoy. The "float" of Ianthina, which 

 attaches the animal to the surface of the water, is probably ho- 

 mologous with the byssus §, judging from its ventral position ||. 



* " Memoire sur le Systeme nerveux de YHaliotide " (Ann. des Sc. Nat. 

 Zoolog. 1858, ser. 4. vol. xii. p. 226). 



t Morch, Journal de Conchyliologie, 1863, p. 39. 



X The double-eyed monstrosities of Emaryinula and Patella vulgata, 

 the latter of which, with a double tentacle, described by Fischer, are not 

 without importance for this comparison (Journal de Conchyliol. torn. v. 

 p. 230, torn. xii. p. 89). Lacaze-Duthiers, "Sur les Monstres doubles de 

 la Bullaa aperta " (Compt. Rend. Acad. Sc. tome xii. 1855, pp. 124/- 

 1250). 



§ Morch, Journal de Conchyliologie, 1860, Juillet. 



Macdonald, '"On the Homologies, &c." (Proc. Linn. Soc. vol. v. 

 Nov. 14, 1860, p. 209). 



|| In the young Cyclas the byssus has, however, a posterior position. 



