65 



Monograph on the Genus Stenochiton (Order 

 polyplacophora), with descriptions of two 

 new species. 



By Edwin Ashby. 



[Read May 9, 1918.] 



Plates XIII. and XIV. 



The genus Stenochiton was formed by Adams and Angas 

 in 1864 (Ad. and Ang., P.Z.S., 1864, p. 193) for the reception 

 of the South Australian shell described by the same workers 

 under the specific name of juloides. 



The characteristics of the genus of which S. juloides was 

 the type are enumerated by Adams and Angas as follows : — 

 "Shell elongated, narrow, convex ; valves longer than wide, 

 not carinated ; apex of the posterior valve subcentral ; plates of 

 insertion multifissate in the end valves, the intermediate valves 

 having 5 fissures on each side ; girdle covered with very minute, 

 polished, imbricating scales." Carpenter (MS.) refers the then 

 only species known to his own later group Stenoradsia, but 

 Pilsbry (in Man. of Con., vol. xiv., p. 55) says: — "Steno- 

 chiton, however, seems to have as much individuality as most 

 of the divisions Ischnochiton, and may be allowed to stand as 

 a subgenus." He somewhat modifies Adams and Angas' 

 characteristics, retaining the following as the distinguishing 

 characteristics of the subgenus : — "Shell very much elongated, 

 roundly arched, valves ischnoid, the central valves having 

 several slits; girdle having minute, smooth, imbricating scales." 

 As will be shown later, some species that evidently should be 

 placed in this genus only show one slit in the central valves, 

 and in one the scales are minutely striated. 



Since Pilsbry wrote the above, two additional species 

 have been described. One by W. T. Bednall in 1897 (Proc. Mai. 

 Soc, vol. ii., pt. 4), under the specific name of pilsbryanus. 

 The habitat is given as Troubridge Shoal, Gulf St. Vincent, 

 "on seaweed ( 1) Zostera." A third species was described by 

 myself under the name of pattens in 1900 (Trans. Roy. Soc. 

 of S.A., 1900). While I have every reason to believe that one 

 or other of the two species described in this paper as new under 

 the respective names of posidonialis and cymodocealis must 

 have been the form described by Mr. Bednall, his description 

 will not coincide with either of these very distinct forms. 

 Either his figures and descriptions are at fault, or he described 

 a fifth species that I have not yet been able to identify. 



D 



