64 



posės bien reguliėrement de chaque cote du limbe, qu'il soit revėtu 

 ou non d'ecailles, d'epines, ou mėme de poils. 



"2. La disposition des branches comraencant plūs ou moins en 

 arriėre et se terminant plūs ou moins en avant. 



" 3. La forme de valves de la coquille, considėrėe specialement 

 dans rexistence plūs ou moins marquė des aires latėrales. 



" 4. La grandeur proportionnelle de ces valves et leur dėgrė 

 d'occlusion. 



" 5. La forrae des lames d'insertion et le nombre de leur ėchan- 

 crures ou dents. 



" 6. Enfin la disposition des couleurs de la coquille." — D. S. N. 

 xxxvi. 536. 



Certainly this \vas a great improvement to what had been pre- 

 viously done, but unfortunately M . De Blainville appears to have had 

 the opportunity of obsendng only a limited number of species, and 

 has placed the others in the sections to which, from their external 

 appearance, they appeared to belong, though on examination they 

 have not the characters assigned to the division in which they were 

 placed : thus Chiton amiculatus, p. 546, is said to have the front and 

 hinder valves lobed and pectinated ; C. niger, p. 541, the teeth of 

 insertion pectinated ; C. echinatus, p. 550, the anterior and posterior 

 valve toothed ; and C. gigas, the lobes not pectinated. 



Fiom repeated examination and comparison I am inclined to con- 

 sider the following as the best characters for the distinction of the 

 genera and species, arranged according to their permanence and im- 

 portance . — 



1. The presence or absence of the pores, furnished with a bundle 

 of spicula on each side of the man tie. 



2. The comparative length and position of the gills. 



3. The form and modification of the plate of insertion of the valves, 

 especially of the posterior valve. 



4. The size and form of the exposed part of the valve, and the 

 kind of sculpture on its surface. 



5. The absence or presence of appendages on the mantle, and the 

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

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

 alvirays 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 platės of insertion can be easily discovered by 

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



