98 Mr, W, H. Benson on the Subgenus Corilla, 
dorsal plates, from the collection of Mr. Gray; 1 6, a plate, 
seen from the anterior side; 1 ¢, half plate, enlarged; 1 d, com- 
plete specimen, hypothetically restored and slightly enlarged. 
Fig. 2 a, Chiton Wrightianus, De Kon., nat, size, showing two dorsal 
plates compressed; 2, dorsal plate, seen on the posterior side ; 
2e, restored specimen, taking as a base the Chiton Loftusianus, 
XVI.—Notes on the Subgenus Corilla, H. & A, Adams; and on 
the Group Plectopylis, Benson; also on Pollicaria, Gould, and 
Hybocystis, Benson. By W, H. Benson, Esq. 
Wiru reference to the group Plectopylis, published in the 
‘ Annals’ for April last, I have received from Mr, Augustus A. 
Gould of Boston, U.S., a sheet containing ‘ Shells of the North 
Pacifie Exploring Expedition,’ with a proposed amended deserip- 
tion of Messrs. H. and A. Adams’s subgenus Corilla. 
* Subgen. Corilla, H. & A. Adams (emendatum).—Testa planor- 
boidea, plerumque sinistrorsa, plus minusve distorta, arcte spirata, 
subtus concava; fauce in fundo denticulis compressis fere occluso, 
quorum uno seepe 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 Rivolit 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 
lamella, 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. Rivoli and 
its congeners; and Helia 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 
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