70 LECTURES ON MOLLUSCA. 



a flat spiral, with a rapidly enlarging mouth. The hole is behind the 

 outer lip, as in Trochotoma, and is gradually brought forward, the 

 part behind being filled up. The animal must therefore have the 

 power of eating out its anal orifice, as it grows older. The shells are 

 found fossil in the oolites, living in the East Indian archipelago, and 

 in the Gulf of California. The boreal form Puncturella resembles it, 

 but with a plate inside to support the anal siphon which is rather long. 

 The young shell of Glyphis exactly resembles Eimula ; but as the ani- 

 mal grows, it becomes conical ; and instead of moving the hole, it 

 enlarges it where first formed, till at last the whole of the spire is 

 eaten away. The animal is larger than the shell, which is always 

 prettily cancellated, and crenulated at the edge. In Fissurella proper, 

 the spiral nucleus has not been detected, even in very young shells. 

 The animal can be entirely drawn into the shell. In most species, the 

 shape is very constant ; but in some, there is great irregularity, not 

 only in the form of sculpture, but even in the shape of the hole. A 

 curious specimen from Mazatlan has two holes ; and another still more 

 extraordinary one, found in Chili by D'Orbigny, has none. Clypidella 

 has a singular, flat, waved shell, with a narrow key-hole. Macros- 

 cliisma has a slug-shaped body, projecting in front of the shell ; which 

 is oblong, with a very large hole behind. The great Lucapina of the 

 Californian coast has an animal as large as a dinner plate, almost cov- 

 ering a flattened crenulated shell. Fissurellidcea, from the Cape of 

 Good Hope and Tasmania, has a very similar animal and shell, but 

 with a smooth border. The shell of the African Pupillcea ; also cov- 

 ered by the mantle of the animal, has a sharp, smooth edge. 



Another group have the anal orifice in front. Emarginula has a 

 shell like Rimula, but with a slit in the outer lip like Pleurotemaria. 

 The shells are always sculptured, and are from deep water. Fossil 

 species first appear in the Trias. In the group Hemitoma, the slit is 

 very small ; and in Clypidina, it is simply a wave. In the " Duck- 

 bill Limpets," Parmaphorus, the shell is white, and almost covered by 

 the black mantle, under which is an enormous foot : there is only a 

 broad wave for the excretory passage. 



In the remaining families of the Scutibranchs, no tendency has been 

 observed to spiral developments, even in the young shell. There are 

 no fringes to the mantle margin ; and the animal is generally of slug- 

 gish habits, and covered entirely by the shell. The teeth also are 

 formed on a much simpler plan, consisting of a few longitudinal series, 

 of variable form. 



Family GADINIAD.E. 



A small family of shells, from the west coasts of the Old and New 

 World, have characters in common with the SipJionarice, or air-breath- 

 ing Limpets. A groove is seen within, proceeding from apex to margin 

 on the right side, going over the muscular scar. This is probably for 

 the vent, as in the last family. But there is only one gill, placed 

 sideways across the back of the neck ; and the tentacles are funnel- 

 shaped. None of the species are colored. They often adhere to other 

 shells, eating out cavities like the Cap-limpets. The west American 



