INTRODUCTION. 45 



valves placed in pairs against each otlier, so as to form a 

 flattened disc attached by means of a tendinous tube called a 

 peduncle. The first of these orders includes the genera 

 Tubicinella, Coronula^ Platylepas, Clitia, Conia, Elmineus, 

 Catophragmus, Octomeris, Balanus^ Creusia, Nobia^ Savig- 

 nium, Pyrgoma, Adna, Megatrema. The second contains 

 the genera Sepas, Scalpellum, Alepas, Pollicipes, Anelepas, 

 Lithotrya, Ibla^ Conchoderma^ Dichelaspis. 



The study of this class is not properly connected with Con- 

 chology. It is now regarded as a sub-class of Crustacea. 



Class CONCHIFERA. 



The shell of a conchiferous animal is always bivalve, com- 

 posed of two pieces placed opposite to each other, joined at 

 the dorsal margins by an elastic hinge. All true bivalve shells 

 belong to animals of this class ; and the correspondence 

 between the shell and the animal is so true that on examining 

 an empty bivalve shell we can not only determine that its 

 inhabitant belonged to this class, but also decide on the par- 

 ticular order and family in which it should be placed, without 

 seeing the soft parts. 



The first general division of Conchifera is that which results 

 from observing the muscular impressions, or marks made on 

 the inner surface of the valve by the insertion of the adductor 

 muscles. All Conchifera are divided into two orders, as fol- 

 lows : 



First Order, Conchifera Dimyaria. 



Having two adductor muscles, and consequently two im- 

 pressions in each valve. They are separated into the follow- 

 ing families : 

 1. TubicolcB (plates, fig. 44 to 54), having shelly tubes be- 

 sides the valves. This family contains the genera Asper- 



