142 ISCHNOCHITON-RADSIELLA. 



the middle. Girdle having very small, solid, smooth scales. Gills 

 almost ambient. (Cpr.) 



Length 40, breadth 26 mill.; divergence 135. 



Sitka to Puget Sound, 9-18 fms. 



Trachydermon trifidus CPE., Stippl. Kep. B. A. 1863, p. 649; 

 Proc. Acad. Nat. Sci. Phila. 1865, p. 60. Ischnoradsia trifida CPR., 

 MS., and DALL, Proc. U. S. Nat. Mus. 1878, p. 331. 



I have not seen this species. The figure is from a rude sketch in 

 Carpenter's MS. Dr. Ball writes of it as follows : 



This rare and fine species is not particularly handsome, being of 

 -dull and livid colors, but is peculiarly characterized by the straight 

 transverse ribs on the dorsal areas with spongy interspaces, and by 

 the pretty regular division of the lateral areas into three well 

 marked radiating costse, which are separated in the insertion plate 

 by two fissures. No other species of the region resembles this in 

 sculpture. Muzzle with a pectinated margin in front produced into 

 rounded lappets at the corners. Gill rows as long as the foot, con- 

 taining each 28-35 branchise. Veil absent. Mantle edge plain, 

 narrow. There is a small spherical lump on each side of the girdle 

 just behind the posterior ends of the gill-rows, which are turned out 

 toward the girdle and widely separated behind. The anus is large 

 median, and crenate, opening on the upper part of the hinder end of 

 the foot. No ovarian openings could be detected, and the species 

 presents some peculiarities which call for further research with more 

 material. (Dall.) 



I. REGULARIS Carpenter. PI. 18, figs. 41-46. 



Shell oblong, elevated, dorsally carinated, the side-slopes straight. 

 Surface appearing almost smooth to the naked eye. Color a uni- 

 form olive or slaty-blue, the girdle having more or less of a blue or 

 purple " bloom." 



The lateral areas are very little raised, and are sculptured with 

 numerous delicate radiating threads, occasionally branching toward 

 the lower margin, and freely branching along the posterior edge of the 

 valve. The terminal valves have similar delicate radii. The central 

 areas have numerous longitudinal, somewhat beaded threads, 

 separated by flat intervals. The posterior valve is elevated, with 

 anterior umbo. 



The interior is light blue. Sutural plates low, connected across 

 the sinus by a narrow plate which is sulcate above and cut into 



