202 



made their usual appearance about the end of January ; and after 

 two or three weeks a considerable number of medusoids were de- 

 tached, of which a few only have survived ; but some of these now 

 exhibit all the specific characters of the perfect Medusa, the largest 

 specimen at the present time being 3 inches in breadth when dilated, 

 and the others of various intermediate sizes. It is unnecessary to 

 detail here the gradual changes undergone in the course of deve- 

 lopment to the perfect animal, as they have been fully and accurately 

 described by several authors : I will only mention that an instance 

 was observed of two medusoids having been thrown off together from 

 the parent stock, and remaining united for more than a week ; each 

 gave evidence of independent existence ; and their course through 

 the water was marked by great irregularity, from the uncertain and 

 sometimes opposite action of the two disks. 



The water containing these Medusse has remained for several 

 months unchanged, but its purity has not been endangered by the 

 presence of fish, or other animals requiring a large supply of oxygen. 



2. On a New Species of Mollusk of the Genus Scisstjrella, 

 d'Orb. By S. P, Woobward, F.G.S. Communicated by 

 Prop. Owen. 



(Mollusca, PI. XL VI.) 



The little shell for which I propose the name of Seissurella 

 mantelli was found in a sample of " Menaccanite " sand, collected 

 in New Zealand by Mr. Walter Mantell. This sand also afforded a 

 minute species of Ringicula, and numerous examples of Calcarina, 

 Siderolites, and other Rhizopods. Seissurella mantelli resembles 

 the type of the genus, S. elegans, d'Orb., but is rather larger, more 

 depressed, more strongly ornamented, and has a longer scissural 

 band. The specimen has been in my possession several years, but 

 I did not think it worth publishing until I observed that it exhibited 

 a character hitherto omitted in all descriptions of the genus, viz. 

 that the shell when young has no slit. M. d'Orbigny's figures of 

 Seissurella elegans, elaborate and highly magnified, represent the 

 scissural band winding round all the whorls and extending to the 

 extreme apex * ; but on referring to the specimens collected by Mr. 

 Jeffreys at Spezzia, I found that the band really terminated within 

 half a whorl of the aperture — a smaller proportional distance than in 

 S. mantelli, and that during the first part of its life the Seissurella 

 elegans also had a simple, entire lip, like the ordinary Trochidce. 



M. d'Orbigny's figures and description are faulty in a still more im- 

 portant respect ; for Mr. G. Sowerby has observed that in the adult 

 shell the fissure became closed at the aperture, leaving only a small 

 foramen ■f : so that Seissurella presents three phases of growth — 

 having a simple aperture when young, a slit when half-grown, and a 



* Memoires cle la Societe d'llist. Nat. de Paris, 1823, pi. 23, p. 340. 



t Zool, Journ. 1824, p. 255, and ' Genera of Becent and Fossil Shells,' fasc. 35. 



