LECTURES ON MOLLUSCA. 217 



characteristic of the Chazy limestone, in which the solid operculum 

 has an upright support, as in Jeffreysia. It is supposed by some to 

 he related to Bellerophon. It is very difficult to determine the rela- 

 tions even of recent shells, when the animal has not been seen, because 

 the shells of such different mollusks are very like each other. Much 

 less can we expect to understand the relations of abnormal fossils, 

 when even the texture affords no clue, and the peculiarities of the 

 mouth can be so seldom examined. 



Family Haliotid^e. {Sea-ears or Ormers.) 



The very beautiful-group of ear-shells may be regarded as Turbos flat- 

 tened out to adhere to rocks. They present however several charac- 

 teristic differences of structure. There are two gills and two auricles, 

 instead of one as in the Top-shells ; and the foot is greatly dilated 

 and very strong. They adhere so tightly to the rocks that they are 

 often forced off by the point of the bayonet. The best way to loosen 

 them is to pour warm water on, and then jirk them with the foot. 

 They are often cooked ; and the shells, which present a very brilliant 

 nacre, golden, green, orange, pink, &c, according to the species, 

 form a regular article of trade for ornaments and inlaid work. The 

 muscular attachment, instead of being horseshoe-shaped, as in ordi- 

 nary univalves, is round and central as in the oyster. There is always 

 a ridge along the back, with a few holes near the edge. These are 

 filled up as new ones are made. Below them is a slit in the mantle 

 to correspond. The foot is very elegantly fringed, and the teeth are 

 complicated as in the Top-shells. The Haliotis tribe are rare in the ' 

 tropics ; but abound in Japan, California, and Australia, and are 

 found along the east coast of the Atlantic. Their absence from the 

 whole of the South and tropical America and the eastern shores of 

 North America, is very remarkable, seeing that they abound from 

 Kamtschatka to Cape St. Lucas. The shells of Padollus have a second 

 spiral rib, but without perforations. In Teinotis, (the Ass's Fars,) 

 the shell is thin and glossy ; the animal being very active, with a large 

 foot. It is thought that the number of holes is constant in each spe- 

 cies ; but this is very far from being the case. In the Californian spe- 

 cies, they vary from two to four, and from five to ten. 



Family Fissurellid^e. (Key-hole Limpets.) 



In this large and beautiful family the body is symmetrical, and 

 only spiral in the first stage. There are two gills at the back of the 

 neck, one on each side of the shell, the vent being between them. 

 This discharges, in the sea-ears, into the last hole: in this family into 

 a hole or slit which is variously situated in the differeift genera. The 

 foot is large and more or less fringed, as in the preceding families ; 

 but the shell is not pearly, and there are no eye-stumps. As in all 

 other Limpets, (with which however they have not a very close connec- 

 tion,) the muscle is horseshoe-shaped. The teeth are arranged in 

 complex patterns, as in the preceding groups. They are found on all 

 shores, though sparingly. The largest species are from South America. 



The shell of Jiimtcla is nearly related to Scissurella, but is formed in 



