2 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



tively small number of low, flat, smooth, even ribs, and the 

 proportionally much smaller girdle scales, which become even 

 more minute toward the margin, are without any undue outer 

 thickening or carination. In passing it may be noted that the 

 so-called "Ischnoradsias" of Northern Japan can evidently be 

 associated neither with the Australian species nor with 

 /. triMus. This phase of the matter will be treated more in 

 detail upon a subsequent occasion in connection with a dis- 

 cussion of the radular characters. 



Ischnochiton regiilaris (Carpenter 1855) was referred by 

 Pilsbry ('92, p. 142) to his section Radsiella, evidently be- 

 cause of its radsioid valves. But Radsiella, as he has it, is 

 an even more heterogeneous assortment than Ischnoradsia, 

 the species referred to it including not only /. tridentatus Pils- 

 bry from Lower California as type, and /. regidaris, but also 

 /. viriduliis (Couthouy) from Tierra del Fuego, the /. trifidiis 

 above mentioned, and /. tigrinus (Krauss) from the Cape of 

 Good Hope. I am not sure that the actual degree of rela- 

 tionship is sufficiently close for the association of any two of 

 these in the same group : certainly the three West American 

 forms have each striking peculiarities to distinguish them. 

 Quite recently Thiele ( :10, p. 112, 113) has shown that the 

 radular characters of regidaris indicate an affinity with the 

 otherwise very diff"erent appearing Lepidozona and Callisto- 

 chiton. This fact, together with the multiplication of inser- 

 tion slits and other shell characters, causes me to propose here 

 a second new group, Rhombochiton, to rank near or under 

 Lepidozona, with /. regidaris (Carpenter) as type. On my 

 labels I am retaining both of these groups under Ischnochiton 

 until certain further investigations upon which I am engaged 

 can be carried through to completion. I do not think that our 

 present knowledge of the detailed morphology of either the 

 shell or the radula can be implicitly trusted to provide the key 

 for working out the true relationships of all these groups, so 

 as conservative a treatment as possible seems now the safest 

 to adopt. 



Through the kindness of Professors J. C. Merriam and 

 B. L. Clark of the University of California, there have been 



