Mr. W. II. Benson on the Genus Opisthoporus. 13 



of a month, I ascertained that these usually sluggish mollusks 

 occasionally swam, at an early hour of" the day, resupinate at 

 the surface. I am not aware that this hahit has been remarked 

 in the genus in question by any other observer. 



I find the following description of the animal of an unpub- 

 lished Physa, which I took, in 1816, at iMichelville, between 

 Cape Town and Hottentot Holland: — 



Foot narrow, hinder extremity pointed, not extending beyond 

 the summit of the spire. Head with veliform lobes or expansions 

 in front. Tentacula subulate, lengthened, and somewhat spread 

 all round at the base ; the eyes being situated on these promi- 

 nences, between the tentacula. Mantle reflected so as to cover 

 merely the edge of the aperture all round, exposing the whole of 

 the breathing cavity, at the hinder part of which, near the junc- 

 tion of the outer lip of the shell with the body-whorl, appears a 

 tongue-like process. 



8th December 1854. 



III. — Characters of the Genus Opisthoporus, an Eastern form of 

 the Cyclostomacea, with Remarks on its Affinities and Notes 

 on several Opercula. By AV. H. Benson, Esq. 



On my passage through Zurich, last summer. Professor Mousson 

 kindly afforded me an opportunity of comparing a shell trans- 

 mitted to me by Dr. Traill from Borneo* (identical with Cyclo- 

 stoma [Pterocyclos) Charbonnieri, Reel., and Cyclotus Taylorianus, 

 Pfr.), with the imperfect specimen of Pterocyclos biciliatuSy 

 Mouss., figured in the ' Mollusken von Java.' We found that 

 they were in nowise to be distinguished from each other. I 

 have long dissented from the received location of this, and allied 

 species, in Cyclotus and Pterocyclos, with reference not only, in 

 the case of the former, to the springing of a retroverted tube 

 from the suture, but also to the formation of the operculum, 

 which, externally, has some resemblance to the vertebra of a 

 fish ; or, as remarked by Recluz, is formed like a pulley, the 

 broad edge of the disk being grooved or excavated in the direc- 

 tion of its circumference. 



On a closer examination it will be found that this apparently 

 solid operculum is formed by two layers, an inner and an outer 

 one, the former having a horny coating : these two layers are 

 united by an erect, internal, spiral lamina, the spaces between 

 which are hollow and hermetically closed ; and the concavity of 



* Vide ' Annals' for 185.3, vol. xi. N.S. p. 32 and 33. 



