UNIVALVES. 6l 



is long in Fusus, the large smooth whelk, and in Trophon, and it is 

 short in Purpura. Purpura lapillus, the dog-whelk, is a massive 

 little shell, not unlike a periwinkle in its typical form, with an oval 

 mouth and thin, deeply-cut, short canal. Periwinkles vary, and so 

 do dog-whelks ; in fact, the variations of the latter are so numerous 

 as to be nameless, and, in some cases, unrecognisable as belonging 

 to the species in the absence of the intermediate forms. The other 

 Purpura is a much more gentlemanly shell, and is of graceful build, 

 with a rich orange mouth ending in a shorter canal than lapillus. 



Though Trophon has prominent ribs, and Fusus has not, the 

 readiest means of distinguishing between them is by their size. 

 Trophon is never over an inch in length, Fusus is never under an 

 inch, and in some species runs up to five inches. The specimen of 

 Fusus antiquus figured in the coloured plate, which is of the variety 

 alba, is just five inches long, and the smallest species, propinquus, 

 with the turreted spire, is well over an inch and a half. Let us note 

 this further step in the usual form : 



Canal straight and open 



Pillar folded or tuberculated Triton. 

 Pillar smooth. 



Canal short Purpura. 



Canal long. 



Shell under an inch Trophon. 

 Shell over an inch Fusus. 



We have only two genera left, both represented by only one 

 species, and both belonging to the same family, though very unlike in 

 appearance, and being the only British representatives of that 

 family. They are brought together here simply because in each 

 case the shell has an indistinct notch, which prevented them being 

 dealt with in our first series. The first, Trickotropis, is pointed at 

 both ends, with a mouth expanding upwards, and has prominent 

 spiral ribs that are cut across by thread-like striations ; the other, 

 Torellia, is not unlike a rounded periwinkle in shape, with a mouth 

 expanding downwards, and having no ribs, but being finely striated 

 in both directions. With Torellia we have the last of the eighteen 

 British genera that have the mouth interrupted by a notch or canal. 



This is not all the British shells. There is another series among 

 the opisthobranchs, some of which have shells, while others have 

 not. In three genera, Philine, Aplysia, and Pleurobranchus, the 

 shells are ear-shaped and very thin ; in Philine the shell is white, in 

 Aplysia it is broad, horny, and glossy, in Pleurobranchus it is broad 

 and iridescent. In two genera it is convolute, Cylichna having the 

 spire hidden, and Tornatinus, otherwise Utriculus, having the spire 

 exposed. In two species the shell is almost convolute, these being 

 Scaphander, with it spirally striated, and Acera, with it swollen and 

 thin. In two cases the shell is oval in general outline, these being 

 Actceon, in which it is spirally striated, and Bulla, in which the spire 

 is involute. Both Action and Scaphander have substantial shells of 

 ordinary appearance, though, in the case of Scaphander, of peculiar 



