68 IM IIN<< IIIToN. 



It is therefore obvious that n<> separation can he made from other 

 suhueiiera of hchnochiton on this character. 



Several species of Trachydermon liave distinctly spon_ 

 like tlinse of T"/tir>//*i. This structure is exceptional in 7.W/M*- 

 chiton where the eavee a- a rule, aie solid, as in the genus Cluito- 

 i. The fact i>e that certain forms of Trachyderinon, 



such a< T. ruber, form a partial transition from hchnochiton to 



/A/. 



From ZVoMyrfermon, Carpenter has sundered a few species hav- 

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



name Ti: \< H \n\ ii \. 



Section Trachydermon, s. sir. 



I. CINEREU8 Linn.'. PI. <i, figs. 25-31. 



Shell oblong, elevated, the dorsal ridge subangular. 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 

 <l)li(jue 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 nit>ile. 

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

 valve 10 or 1 4 J. The teeth of the posterior valve are rugose. The 

 intermediate valves have a single slit on each side. 



lor varying greatly, the most common pattern being a.-hcn 

 thickly mottled and blotched with olive ; others are a li.irht fleshy- 

 oranire. and frequently the light and dark patterns are combined on 

 ;irioiis valve- of one individual. 



The girdle has usually alternating light and dark bars. It 

 1 with minute >e;il.-s th- edge having a fringe of delicate 

 v .-pines. 



