LECTURES ON MOLLUSCA. 241 



little-known family are extremely minute. It is very probable that 

 the animal of Gheletropis will be found closely allied. 



Family Atl antics. 



The beautiful little glassy Atlanta, when first discovered, was sup- 

 posed to be a recent Ammonite. It has a flat, keeled shell, very sharply 

 keeled, and deeply notched like Scissurella. The broad, triangular 

 swimming-fin has a little disc, with which it can moor itself to any 

 floating object. The operculum begins as a right-handed spiral, but 

 continues straight. The snout is very long ; the eyes and tentacles 

 large, and the neck thin. Oxygyrus has a cartilaginous shell, with a 

 triangular, concentric operculum, like the supposed opercula of Am- 

 monites. The teeth have a general similarity to those of Garinaria. 



Family BELLEROPHONTIDiE. 



The Bellerophons are a singular race of ancient fossils, the true affini- 

 ties of which are not yet agreed on. They are thin, globular, spiral 

 shells; like a Nautilus, but without chambers, and displaying a keel or 

 notch in the middle. Some liken them to Argonauts; others to Bullas; 

 others consider them as enrolled Emarginidas ; but the best-supported 

 opinion is that they are as it were swollen Atlants. The little cre- 

 taceous species, without notch, are called Bellerophina. The paleozoic 

 species with the whirls exposed are Bucania. Those with the whirls 

 scarcely embracing, like an unchambered Ammonite with a slit mouth, 

 are Porcellia. In Gyrtolites the whirls do not touch, and in Ecculiorn- 

 phalus they are drawn out like Spirula. 



Family Pirolidje. (Glass- Argonauts.) 



It is no wonder that the shell of Garinaria has been taken for an 

 Argonaut; and even that the true animal of the Argonaut was thought 

 to be allied to this, which may be considered as the typical Heteropod. 

 The front part of the gelatinous body is enormously developed, while 

 the abdomen is small, and the tail (which takes the place of the oper- 

 cular arm of the Atlants) is short and pointed. There is along snout ; 

 with a short tongue, toothed as in the Strombs and Helmets. The 

 eyes are hour-glass shaped, highly organized, and often furnished with 

 a little eyelid. They float upside down, with their foot at the top, in 

 the shape of a flat fin, armed with a small sucker for adhesion. Below, 

 the principal viscera hang out from the back, and are protected by the 

 glassy shell, the gills projecting beyond it. They come up to the 

 surface to feed in the evenings, and are found in most warm seas. 

 Gardiapoda has a discoidal shell, with flaps round the mouth. In 

 Firola, there is no shell to protect the nucleus : and in Firoloidea, the 

 gills are on the tail, and there is no sucker on the fin. 



Family Phylliroice. 



This family may be considered either as degraded Heteropods or 

 Nudibranchs, forming an exact transition between the two. They 

 16 



