486 



CALIFORNIA ACADEMY OF SCIENCES 



[Proc. 4th Ser. 



Remarks: Occasional unmistakable specimens of this pe- 

 culiar species have been discovered in the Pleistocene of many 

 localities, and in the Long Wharf Canyon deposit it is one 

 of the commonest forms to be found, being there exceeded in 

 abundance only by the nearly related C. palmulatus mirabilis 

 Pilsbry. This seems the more strange, since nowhere along 

 our coast at the present time can it be said that crassicostatus 

 is found in any very appreciable numbers. Usually but an oc- 

 casional specimen or two is taken even at extreme low tides. 

 Along the entire coast of southern California the commonest 

 present-day Callisto chiton is by all odds C. decoratus puncto- 

 costahis, a species for from common as a fossil. 



Although very distinct in the living condition, poorly pre- 

 served fossils of crassicostatus are sometimes hard to identify 

 on account of the possibility of their being mixed with C. pal- 

 mulatus mirabilis. Well preserved valves, on the other hand, 

 are as readily separable as recent specimens. This is quickly 

 seen when the chief diagnostic characters of the two forms 

 are placed in parallel columns : 



HEAD VALVE 



crassicostatus 



Primarily with 7 heavy, irregu- 

 larly tubercular ribs, each divided 

 almost at once by a median sulcus, 

 and in old shells with a secondary 

 sulcus on each side toward the base. 



Slits typically 9 in number, 2 on 

 each side of the central slit being 

 adnatc. 



mirabilis 



Primarily with 9 moderately 

 heavy, strongly tubercular ribs, each 

 sooner or later showing in the adult 

 a median sulcus; the posterior rib 

 on each side apparently duplex. 



Slits typically 9 in number, regu- 

 larly spaced. 



MEDIAN VALVES 



Lateral areas elevated into a 

 typically 20 single, very heavy, crudely 

 nodulose rib, with usually about 4 

 rather weak, radiating furrows. 



Lateral areas divided by a strong 

 median furrow into two nodulose 

 ribs, each of which may bear a 

 weak secondary furrow. 



Central areas with 7-12 quite 

 strong, interlatticed ribs on a side, 

 showing a distinct tendency to con- 

 verge anteriorly on the ridge, even 

 on valve ii. 



Central areas with 14-18 (fewer 

 in very small specimens) moderately 

 strongly interlatticed ribs on a side, 

 not showing any well-marked tend- 

 ency to converge in front, and 

 strongly divergent on valve ii. 



20 One fossil specimen [682], which seems otherwise thoroughly referable to crassi- 

 costatus. shows sharply duplex lateral ribs recalling those of palmulatus and mirabilis. 



