56 ISCHNOCHITON. 



Carpenter gives the following notes on the type specimen : Length 

 22 , breadth 4? mill. ; shell very narrow, elongate, elevated, cylin- 

 drical, the jugum rounded ; red-brown, narrowly streaked with 

 paler. Posterior valve elongated, mucro median, scarcely conspic- 

 uous, the slope behind it straight. Anterior valve forming more 

 than a semicircle, its posterior margin (suture) emarginate. Entire 

 surface delicately granulated in quincunx; jugal area scarcely 

 defined ; lateral areas conspicuously elevated. Girdle inconspic- 

 uous, covered with minute imbricating scales. Scales almost all 

 destroyed, but looking like those of /. sanguineus, etc. Sinus 

 large, very deep, flat, wide ; sutural plates much elongated, narrow. 

 Central valves regularly 3-slit ; posterior valves about 16, anterior 

 valves about 13 slits. The slits are irregular, sometimes bifurcating 

 but typically rather distant. Teeth very acute, moderately long. 

 Eaves moderate. 



Subgenus II. STENOPLAX Carpenter, 1878. 



= Stenoplax Cpr. + Stenoradsia Cpr. -f- Manger ella Cpr. 



Elongated, narrow Ischnoids, having the girdle covered with 

 minute, crowded, imbricating striated scales. Median valves having 

 the lateral areas much raised; posterior valve large, depressed, the 

 mucro subcentral or slightly posterior. 



The subgenus Stenoplax (-}- Stenoradsia and Maugerella) has a 

 very characteristic peculiarity. The sides and front end of the foot 

 are so much dilated that the head of the animal is nearly or entirely 

 concealed from below, the thin, produced, anterior end of the foot 

 extending quite over it. The gill row is shortened at both ends, 

 reaching nearly to the head, but shortened at the tail about one- 

 fourth the entire length of the foot. The branchiae are largest at 

 the middle. 



STENOPLAX divides naturally into two sections: (1) Section 

 Stenoplax (restricted), in which the median valves have a single 

 slit on each side, and (2) Section Stenoradsia, the species of which 

 have two or several side slits in the median valves. 



Section Stenoplax Cpr. (restricted). 



Stenoplax CPR., MS. and table of Regular Chitons, 1873. DALL, 

 Proc. U. S. Nat. Mus. 1878, p. 330, type J. limaciformis Sby. 



This section is closely allied to Stenoradsia, the latter differing 

 only in having several slits in the plates of the intermediate valves. 



