Zoological Society. 69 



form, sculpture on the surface, disposition, and equality or inequality 

 of size of these appendages. 



6. The colour of the valves and appendages of the mantle. 



It has been objected, that the character derived from the form of 

 the plates of insertion can only be seen by the destruction of the 

 specimens, as they are generally kept in the cabinets : this is not 

 always the case, for they can generally be seen from the under-side 

 or through the substance of the mantle ; but when this is not the 

 case, the form of the plates of insertion can be easily discovered by 

 carefully paring away the under part of the mantle, so as to show part 

 of the edge of the valve without any injury to the specimen. And 

 it should be recollected too, that the separate valves are the only part 

 of the molluscous animals which are usually kept in cabinets. 



The number of lobes into which the edge of the margin of inser- 

 tion is divided may be also easily seen by the porous lines which are 

 to be observed on the inner surface of the valves, diverging from the 

 apex to the margin, each of these lines going to the bottom of the 

 notch which separates the lobes on the inner processes of insertion. 



Various authors, as Spengler, Chemnitz, De Blainville, Sowerby, 

 Barnes and Reeves, have described and figured many species of the 

 genus. 



Synopsis of the Genera. 



I. Mantle simple, without any pores or tuft of spines on the sides. 



A. The plate of insertion of the anterior and posterior valve divided 

 into several lobes, and of the central valves into two lobes. 



a. The valves exposed, broad, with regular, equal, well-defined mar- 

 gin for insertion, divided into lobes more or less denticulated. The 

 hinder valve with the apex superior, subcentral. 



1. Chiton. The posterior valve entire; margin covered with 

 regularly- disposed imbricate scales. 



2. Tonicia. Posterior valve entire ; margin naked. 



3. Acanthopleura. Posterior valve entire; margin spinose, spi- 

 nulose or bristly. 



4. Schizochiton. Posterior valve with a deep notch on its cen- 

 tral hinder margin ; mantle slit behind. 



b. The valves exposed, broad; the hinder valve with a slightly raised, 

 smooth or slightly crenated plate of insertion (not divided into lobes 

 on the sides), and with the apex subterminal. 



5. Corephium. The hinder valve with a rather raised apex, and 

 the plate of insertion crenulated, with one small central slit. 



6. Plaxiphora. The hinder valve with a produced posterior 

 apex, and the plate of insertion entire, smooth, rounded ; valves thin ; 

 mantle with tufts of bristles. 



7. Onithochiton. The hinder valve with a produced terminal 

 apex ; plate of insertion entire, rounded ; valves thick ; mantle covered 

 with spines, bristles, or chaff-like scales. 



8. Enoplochiton. The hinder valve with a produced terminal 



