77 



commencing quite shallow at mucro and increasing in depth 

 until the anterior margin of valve is reached at the girdle. 

 The slope behind mucro almost straight but slightly concave 

 near girdle. 



Median valves. — Lateral area more strongly raised than 

 is the case in any other known species of Stenochiton. This 

 area is, in common with the rest of the shell, highly polished, 

 the only sculpture being several concentric grooves following 

 the growth-lines. The strong diagonal ridge which divides 

 this area from the pleural area is formed by a rapid drop 

 from the anterior margin to the pleural area, and not by 

 any definite raising of the lateral area. 



Pleural and dorsal areas. — Indistinguishable from one 

 another, except that the latter is usually outlined by a 

 dark V-shaped mark. Both areas are smooth and highly 

 polished, but the pleural area, where it abuts on the lateral, 

 has the appearance of being broadly hollowed out, thus add- 

 ing to the abruptness of the separating ridge that forms the 

 anterior margin of the lateral area. Slits 3 to 4 very narrow 

 (Carpenter gives central valve 3 slits, Adams and Angas- 

 5 slits). This character seems rather specific than generic. 

 Girdle narrow, clothed with small, closely-packed, imbricating 

 scales, which are more flattened than in Stenochiton 

 cymodocea, and although straight-sided are almost as broad 

 as long. The scales in the outer row are lengthened and 

 pointed, under 1-inch objective resembling short blunt 

 spicules. The effect of a fringed edge is hardly perceptible. 



Measurements. — Total length of dried specimen not dis- 

 articulated, 40 mm. Length of valves taken longitudinally 

 at suture: (1) 5 mm., (2) 3 mm., (3) 4 mm., (4) 4'25 mm., 

 (5) 5'5 mm., (6) 6 mm., (7) 5'5 mm., (8) 5'5 mm. Breadth 

 of valves: (6) 6'5 mm., anterior valve 4 mm., posterior valve 

 nearly 6 mm. It will be seen that the shell is widest at the 

 sixth valve, and tapers forward to the anterior valve. The 

 tapering of the posterior valve is rapid, as has been before 

 alluded to. 



Remarks. — The shape of the anterior valve, great length 

 of the shell, and the raised lateral area with its abrupt ridge, 

 easily distinguish this species. They are rather specific 

 differences than generic characters. 



Note. — While I have in the choice of distinguishing 

 names departed somewhat from the fashion that has mostly 

 been followed heretofore by workers in Polyplacophora, I 

 deem no apology is necessary. The striking habit® of the group 

 dealt with suggest that their names should be chosen with 

 reference to them, rather than to their valvular structure. 



