68 ISCHNOCHITON. 



It is therefore obvious that no separation can be made from other 

 subgenera of Ischnochiton on this character. 



Several species of Trachydermon have distinctly spongy eaves, 

 like those of Tonicella. This structure is exceptional in Ischno- 

 chiton where the eaves as a rule, are solid, as in the genus Chceto- 

 pleura. The fact seems to be that certain forms of Trachydermon, 

 such as T. ruber, form a partial transition from Ischnochiton to 

 Tonicella. 



From Trachydermon, Carpenter has sundered a few species hav- 

 ing Radsioid central valves, to which he has given the sectional 

 name TRACHYRADSIA. 



Section Trachydermon, s. sir. 



I. CINEREUS Linne. PI. 6, figs. 25-31. 



Shell oblong, elevated, the dorsal ridge subarigular. Anterior 

 valve half-circular, the posterior margin decidedly emarginate. 

 Very evenly and finely sculptured with diamond-shaped granules, 

 which, although somewhat irregular, seem to be formed by the 

 oblique intersection of curved incised lines radiating in two direc- 

 tions from the apex. Intermediate valves having precisely the 

 same sculpture upon both central and lateral areas, the latter 

 slightly raised, but not distinctly separated from the central areas. 

 The posterior margin of the intermediate valves is rather produced 

 or beaked in the middle. Posterior valve decidedly smaller than 

 the anterior, having the umbo low, inconspicuous, and somewhat 

 anterior. 



Interior whitish or brown-tinged ; sutural plates wide ; sinus 

 narrow. 



Insertion plates nearly smooth, slightly roughened outside. 

 Number of slits in the anterior valve 9 or 10; in the posterior 

 valve 10 or 12. The teeth of the posterior valve are rugose. The 

 intermediate valves have a single slit on each side. 



Color varying greatly, the most common pattern being ashen 

 thickly mottled and blotched with olive ; others are a light fleshy- 

 orange, and frequently the light and dark patterns are combined on 

 the various valves of one individual. 



The girdle has usually alternating light and dark bars. It 

 is covered with minute scales, the edge having a fringe of delicate 

 glassy spines. 



