68 Zoological Society. 



scales so gradually pass into spines or tubercles on the one side, and on 

 the other they so gradually diminish in thickness to furfuraceous 

 scales, which are easily deciduous that it is difficult to define when they 

 are quite absent ; therefore they do not afford characters of sufficient 

 importance to use them as Leach, Risso and Guilding have done, for 

 the primary divisions of the family. 



Lamarck divided the family into two genera, Chiton and Chitonellus, 

 but he left in the former genus several species which are more natu- 

 rally allied to the latter. 



M. De Blainville in 1825 published a monograph of the family, 

 under the article ' Oscabrion' in the Diet. Sci. Nat. xxxvi., in which 

 he introduced some new characters for the division of the species into 

 sections. He observes : " Les organes sur lesquels nous appellerons 

 successivement l'attention pour le distinction des especes sont les 

 suivants : — 



"1. L' existence ou l'absence des paires de pinceau de soies dis- 

 poses bien regulierement de chaque cote du limbe, qu'il soit revetu 

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



"2. La disposition des branches commengant plus ou moins en 

 arriere et se terminant plus ou moins en avant. 



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

 dans l'existence plus ou moins marque des aires laterales. 



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

 d'occlusion. 



"5. La forme des lames d'insertion et le nombre de leur echan- 

 crures ou dents. 



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

 xxxvi. 536. 



Certainly this was a great improvement to what had been pre- 

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

 the opportunity of observing 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. gig as, the lobes not pectinated. 



From 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 mantle. 



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 



