48 INTRODUCTION. 



shells, covered on the outside by a thick horny epidermis, 

 and pearly within. They include the genera Unio, 

 Hyria, Anodon, Iridina. 

 14. Chamacea (plates, fig. 153 to 155), inequivalve, irregular, 

 foliaceous, attached; containing the genera Diceras, 

 Chama, and Etheria. 



Second Order, Conchifera Monomyaria. 



Having one adductor muscle, and therefore only one im- 

 pression in each valve. They are separated into the following 

 families : — 



] . Tridacnacea (plates, fig. 156 & 157), transverse, equivalve, 

 with an elongated muscular impression, near the centre 

 of the ventral margin ; margin undulated at the termi- 

 nation of the radiated large ribs. The genera Tridacna 

 and Hippopus are included. 



2. Mytilacea (plates, fig. 158 to 162), generally regular, with 



the hinge linear, without teeth, occupying the greater 

 part of the dorsal margin. This family includes the 

 genera Modiola, Mytilus, Pinna. 



3. Malleacea (plates, fig. 163 to 170), shell generally thin, 



inequivalve, irregular, foliaceous, with the hinge linear. 

 This family contains the genera Crenatula, Perna, Mal- 

 leus, Avicula, Meleagrina. 



4. Pectinides (plates, fig. 171 to 178). The Pectinides are 



generally regular or nearly so, with the shell solid ; the 

 greater part of them are auriculated at the dorsal mar- 

 gin, and generally characterized by ribs radiating from 

 the umbones. The genera are Pedum, Lima, Plagio- 

 stoma, Pecten, Plicatula, Spondylus, Podopsis. 



5. Ostracea (plates, fig. 180 to 192). The shells of this 



family are irregular, generally attached and foliaceous. 



