98 Mr. W. H. Benson on the Subgenus Corilla, 



dorsal plates, from the collection of Mr. Gray; 1 b, a plate, 

 seen from the anterior side ; 1 c, half plate, enlarged ; 1 d, com- 

 plete specimen, hypothctically restored and slightly enlarged. 

 Fig. 2 a, Chiton Wrightianus, De Kon., nat. size, showing two dorsal 

 plates compressed ; 2 b, dorsal plate, seen on the posterior side j 

 2 c, restored specimen, taking as a base the Chiton Loftusianus. 



XVI. — Notes on the Subgenus Corilla, H. fy A. Adams; and on 

 the Group Plectopylis, Benson ; also on Pollicaria, Gould, and 

 Hybocystis, Benson. By W. H. Benson, Esq. 



With reference to the group Plectopylis, published in the 

 1 Annals ' for April last, I have received from Mr. Augustus A. 

 Gould of Boston, U.S., a sheet containing ' Shells of the North 

 Pacific Exploring Expedition/ with a proposed amended descrip- 

 tion of Messrs. H. and A. Adams's subgenus Corilla. 



" Subgen. Corilla, H. & A. Adams (emendatum). — Testa planor- 

 boidea, plerumque sinistrorsa, plus minusve distorta, arete spirata, 

 subtus concava ; fauce in fundo denticulis compressis fere occluso, 

 quorum uno ssepe ad aperturam producto ; peristomate incrassato, 

 reflexo." 



Mr. Gould adds a new species from Hong Kong, C.pulvinaris, 

 G., with " denticulis in fauce ad 9, haud productis " among the 

 characters of the aperture. This shell he states to be " almost 

 precisely of the size and shape of H. refuga, Gould; but that is 

 reversed, and has a lamina running to the aperture." 



Mr. Gould informs me that in a more extended paper he has 

 gone more fully into individual peculiarities. This was pub- 

 lished, he further states, in 1859. I have not had the good 

 fortune to meet with it; and for more than six months have 

 been in vain endeavouring to get a copy of a paper on Siamese 

 shells, published several years earlier at Boston. 



Now the subgeneric character, "fauce in fundo denticulis 

 compressis fere occluso," seems to provide for the retention of 

 Helix Rivolii and H. erronea, which the characters of Plectopylis 

 absolutely exclude from my group, and leave in Messrs. Adams's 

 original subgenus Corilla, as they are furnished only with spiral 

 lamelhe, and have no pylaic barrier. On the other hand, the 

 character " planorboidea " would ignore H. plectostoma and H. 

 Pinacis, in which the pylaic barrier is present. 



Messrs. Adams's typical species of Corilla are H. Rivolii and 

 its congeners ; and Helix plectostoma had been referred to a 

 distinct group. Plectopylis was designed to unite shells pre- 

 viously referred to different subgenera (although allied by the 

 presence of pylaic barriers), and to separate species destitute of 



