308 PLACIPHORELLA. 



lacking all radiating sculpture. Posterior valve (figs. 84, 85) 

 depressed, the mucronear the posterior margin, which is slightly and 

 rather broadly waved inward. 



Interior delicately blue-green tinted. Sinus a rather shallow 

 rounded wave in the earlier valves, becoming deeper, narrower and 

 angular in the eighth valve. Anterior valve having 8 slits, median 

 valves having 1 slit on each side ; the eaves narrow, tenaciously 

 adhering to the girdle. 



Posterior valve small, its greatest width, including insertion and 

 sutural-plates, measuring hardly more than one-half the width of the 

 widest median valves ; the short posterior insertion-plate rising from 

 a very heavy callous rim ; having one oblique slit on each side, and an 

 almost imperceptible wave where the posterior sinus should be. 



Girdle rather narrow, brown, leathery, sparsely beset with spinu- 

 les (the stumps only remaining in the specimen before me), of which 

 there is one at each suture and a marginal fringe. 



Length 17, breadth 123 mill; divergence 140. 



Hakodadi Bay, Japan (Stimpson), in 25 fms. 



Chiton (Mopalia) stimpsoni GOULD, Proc. Bost. Soc. N. Hist., 

 vii, p. 165 (1859) ; Otia Conch, p. 118. Conf. BALL, U. S. Nat. 

 Mus. 1886, p. 210. 



The above description is from the type in the U. S. National 

 Museum (no. 1646). It will be seen that the species differs from P. 

 borealis in the smooth, not radially sculptured anterior valve, in 

 the strongly calloused interior of the posterior valve, its shallower 

 tail sinus, etc. The P. velata Cpr. of California has the valves very 

 similar to this species in form ; both differing widely from P. bore- 

 alis in the solid, not spongy, sinus, partly filled by a short lamina 

 connecting the sutural plates ; in the more calloused interior of the 

 posterior valve and its smaller size ; the non-radiated anterior valve, 

 etc. 



Carpenter has given the following notes in his MS. on a fine 

 specimen from Japan, in the Jeffreys collection. "As curled up 

 and dried it measures, length 23, breadth 21 mill. ; divergence 120. 

 It is olivaceous and orange-ash variegated. Anterior valve smooth. 

 Posterior valve having the mucro raised, at the posterior third, the 

 sinus continued upward to it outside. Central valves with two 

 obsolete but swollen ribs, one sutural, the other diagonal, with a 

 broad depression between them. The girdle is very much shrivelled 



