18 IXTRODUCTIOX. 



Tellina, Cardiiim, Mactra, Donax, Venus, Spondylus, Chama, 

 Area, Ostrea, Anomia, Mytiliis, Pinna. 



In the third order there are nineteen genera: Argonauta, 

 Nautilus, Conus, Cyprsea, Bulla, Voluta, Buceinuni, Strom- 

 bus, Murex, Trochus, Turbo, Helix, Nerita, Haliotis, Pa 

 tella, Dentalium, Serpula, Teredo, and Sabella. 



It will now be proper, before proceeding to a description 

 of the generic characters of the Linnaean system, to make 

 the student acquainted with the terms used in Testaceology, 

 as far as regards the different parts of shells. 



EXPLANATION OP THE PAETS OF SHELLS. 



MULTIYALVE SHELLS. 



Valves of Chitons. — Chitons in general hav^e eight 

 transverse, broad, but very short valves, placed on the back 

 of the animal, and inserted at their sides into a marginal 

 tough ligament. Plate I. fig, 6, a, a, a. 



Operculum consists of four small valves on the summit 

 of the Lepas, which shut up the superior orifice; it is in a 

 certain degree stationary, and different from the operculum 

 of univalve shells, which will hereafter be described. Plate I. 

 fig. 1, A. Fig. 2. represents a profile view of the operculum 

 removed from its place, A the front valves, B the back 

 valves. Fig 3. a front view of the operculum. 



Base is that part of the shell by which it is fixed to rocks 

 and other bodies. Plate I. fig. 1. and 5, b b b. — h is a piece 

 of stone to which the base is fixed, and g a piece of wood to 

 which the shells of the particular kind here represented are 

 generally attached. 



Ligament is the membraneous or tendinous substance by 

 which the valves or parts of the shell are attached. Some 

 niultivalve shells are connected, by the parts of one valve 

 locking into another. Plate I. fig. 6, a, a, a. The liga- 



