PRELIMINARY OBSERVATIONS. 



xxiii 



a horny substance called an operculum, that serves as a door to close 

 the shell, when the animal withdraws into it. In many species of uni- 

 valves, the animal can fold the mantle so as to form a tube which pro- 

 trudes into the water, while the head and foot remain in the shell. 

 Some species of univalves are carnivorous, others are herbivorous, 

 and the nature of their food determines their residence either near the 

 shore or in deep water. 



Fig. 15. represents the shell and animal of a species of Buccinum, 

 which agrees with the above description of the inhabitants of univalve 

 shells. The foot on which it crawls is on the left hand, with the oval 

 operculum near the end of it. On the right hand of the figure, at the top, 

 the mantle is represented folded, to form a tube, as above described. 



In some species, both of bivalve and univalve shells, the animals 

 depart considerably from the general character of the class to which 

 they belong. There are some bivalves which have the cavities of 

 the shells divided by partitions, the uses of which are not known ; 

 and some univalves have an apparatus for swimming on the surface of 

 the water. 



The Hippurite, a remarkable fossil bivalve, with a deep conical 

 under shell, and a fiat lid, is represented fig. 14. It is classed by Cuvier 

 with the oyster family ; and, by Parkinson, with chambered shells. 

 The nature of the animal is unknown. The shell is divided by trans- 

 verse septa, or partitions, on which account Mr. Parkinson places it 

 among other species of chambered fossils. The existence of a lid 

 seems to prove, that it was not an internal shell, but the habitation 

 of the animal. A fossil hippurite has recently been found in the chalk 

 hills of Sussex, by Mr. Mantell. 



The Janthina is a beautiful purple-coloured univalve shell, nearly 

 resembling in form the snail ; Lamarck discovered, that it could not 

 crawl on its foot, but that the foot is covered with air bladders, which 

 enable the animal to rise and swim on the surface of the water. The 

 janthina is common in the Mediterranean ; when touched, it excretes 

 a deep purple liquor, which tinges the surrounding water. (Cuvier, R. 

 A. torn, iii.) There are other animals occupying univalve shells, that 

 have the power of swimming. The Lymnea stagnalis, an inhabitant 

 of ponds, swims on the surface of the water in a reversed position. It 

 descends by compressing itself whithin the shell, and expelling the 

 air, and thus sinks immediately to the bottom. Mr. Parkinson rightly 

 conjectures, that the shells resembling the Helix, or snail, in the older 

 strata, were constituted for swimming, like the janthina : they could 

 scarcely have used a foot for crawling, at the bottom of a deep and 

 agitated ocean. 



We come now to another division of the animal kingdom, called by 

 Cuvier Radiated. See Chap. II. Some of the animals comprised in 

 this division have left abundant remains in the fossil state, particularly 

 the encrinite and the pentacrioite. These animals had a stem, com- 

 posed of numerous plates, and terminating in branches surrounding 

 the mouth, resembling the stem and branches of a vegetable. Both 

 these species were supposed to be extinct; but a living pentacrinus 

 has been discovered in the West Indies, and a smaller species, more 

 recently, in the Cove of Cork. This has been described by Mr. J. V. 

 Thompson, of Cork. A drawing of this animal, taking by Mr. 

 Thompson, is given (Plate VIII. fig. 17. A cut of a remarkable species 



