PLAXIPHORA. 313 



Subgenus PLACOPHOROPSIS Pilsbrj, 1893 (n. s.-g.) 



Shell short, obovate, the valves short and wide ; having very short, 

 almost obsolete insertion-plates, that of the anterior valve having 

 many (about 20) shallow notches ; posterior valve with callous ridges 

 at the sides and a shallow rounded caudal sinus. Girdle enormously 

 broadened in front, narrow behind. 



This section is intermediate between Placiphorella and Plaxipliora, 

 differing from both and from and all Mopaliidce, in the multiplica- 

 tion of anterior slits, although some specimens of Placiphorella show 

 the same tendency. It must be understood that although in many 

 groups of Chitons, such as all Ischnochitonince and Chitonince, the 

 number of anterior slits is a character of merely specific importance, 

 the case is far otherwise in those groups in which the slits correspond 

 in number and position with external ribs, such as Nuttallina and its 

 allies, and the Mopaliida;, Acanthochitidce, etc. In these groups the 

 number of slits in the anterior insertion-plate is a highly constant 

 generic character, apparent exceptions being generally readily trace- 

 able to the splitting of one or more primary teeth. 



Besides the character just discussed, Placophoropsis resembles 

 Placophorella in the form of the girdle ; but it differs in lacking 

 Mopaloid slits in the posterior insertion plates. Placophoropsis 

 resembles Plaxiphorain the non-slit posterior insertion-plate, but it 

 differs in the obsolescence and multiple slitting of the anterior inser- 

 tion plate, and in the anterior dilation of the girdle. 



The obsolescence or shortness of the insertion-plates in this form 

 is evidently due to degeneration from disuse. Deep sea Chitons not 

 being subjected to the constantly recurring impacts and stresses 

 which affect littoral forms, have no need of elaborate structures for 

 binding the valves in their places. Consequently the primitive 

 forms, such as Lepidopleurus, have been able to survive unaltered 

 under these conditions. In the case of Placiphoropsis, which is 

 descended from littoral forms having long, well-developed insertion 

 plates, especially in the anterior and median valves, the shortness of 

 the plates is to be ascribed to the absence of the external stimuli 

 which act constantly upon the littoral Mopaliidce. 



P. ATLANTICA Verrill & Smith. PL 66, figs. 18-24 ; pi. 65, figs, 73, 



74, 75. 



Shell obovate, wider in front, tapering toward the small posterior 

 valve, which is only about half as wide as the anterior valve. Rather 



