65 



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 Blain\411e, Sowerby, 

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

 genus. 



Synopsis of the Genera. 



I. Mantle simple, tvithout any pores or tu/t of spines on the sides. 



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



into Severai lobes, and of the centrai valves into two lobes. 



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

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

 hinder valve tvith the upes superior, subcentral. 



1. Chiton. The posteri«e valve entire ; margin covered with 

 regularly-disposed imbricate scales. 



2. ToNiciA. Posterior vai* entire ; margin naked. 



3. AcANTHOPLEūKA. Posterfor valve entire ; margin spinose, spi- 

 nulose or bristly. 



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

 trai hinder margin ; mantle slit behind. 



b. The valves exposed, broad; the hinder valve tvith a sUghtly raised, 

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

 on the sides), and tvith the apex subterminal. 



5. CoREPHiuM. The hinder valve -vvith a rather raised apex, and 

 the plate of insertion crenulated, -vvith one small centrai 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 terminai 

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

 with spines, bristles, or chafF-like scales. 



8. Enoplochiton. The hinder valve -vvith a produced terminai 

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

 covered vvith oblong, unequal, elongated, oblong scales. 



B. The plate of insertion of all the valves tvith only a single notch on 

 each side. The valves more or less covered ; the hinder valve tvith 

 expanded platės of insertion (as in the centrai valves), tvith only a 

 single notch on each side, and a concave sinuosity beloto. 



9. MoPALiA. Valves, exposed part broad, transverse ; platės of 

 insertion moderate ; mantle spinulose ; front edge sometimes ex- 

 panded. 



10. Katharina. Valves, exposed part small, cordate, as long as 

 broad ; mantle smooth. 



11. Cryptochiton. Valves entirely hidden ; mantle covered with 

 tufts of spicula. 



