1869.] On the genus Onchidium. 99 



does, however, not stand a long immersion in tank water, and in brack- 

 ish water it dies much sooner. Onchidium tenerum burrows in soft 

 mud, and appears on the surface only in warm weather after the rain. 

 Lesson says of Ochidium ferrugineum, that it is a truly marine spe- 

 cies, living as a rule, several feet below the surface of the water. 



Relations and probable identity of Onchidium with Onchidella and 

 Peronia. 



I have given the anatomy of the type species, Onchidium iyphce, 

 in detail, because it must form the basis of further comparison with 

 other species, which have been believed to belong to distinct generic 

 types. Cuvier, in his admirable " Memoires, p. serv. l'histoire et 

 l'anatomie des Mollusques," 1817, gave a very good account* of the 

 anatomy of Onchidium Peronii from Mauritius, and drew attention 

 to the existence of a small British species, Onch. celticum. Lesson, 

 described several species in the " Voyage de la Coquille ;" 0. granulo- 

 sus,, marmoratum, ater et ferrugineum. Avery good general figure and 

 correct drawings of the different external organs are given by Savigny 

 of the so-called Peronia verruculata, from the red sea, in the French 

 Scientific Expedition to Egypt, (Moll. PI. III). Quoy and Gaimard, 

 in the " Voyage de l'Astrolabe" (Moll. PI. XV) figure five species, 

 but in none of them the position of the genital pores has been noticed. 

 No details of the anatomy are given. Gray refers the largely tuber- 

 culated species, like O. punctatum and Tongensis to Peronia, the 

 granular ones, like O. patelloide and incisum to Onchidella. Several 

 other species of the same group of Molluscs were described by other 

 authors from Mauritius, the Phillipines, etc. Keferstein lately (Zeit- 

 schrift fur wiss. Zoologie, Bd. XV, 1861, p. 76-85) published some 

 notes on Janella, Aneitea, and allied forms, but unfortunately I have 

 not as yet been able to procure this paper. However, as far as the 

 forms which interest us here specially are concerned there is sufficient 

 for our purpose extracted in Bronn's " Klassen and Ordnungen des 

 Thierreiches," Vol. III. On plate 105 a good side view is given 

 of Peronia verruculata, shewing the correct position of the pulmonary, 



* The figures are reversely drawn, for instance in figures 2 and 5 tha 

 external vas deferens appears on the left side, and equally so the heart ia 

 figure 5, which represents an upper view. 



