Vol. XI] BERRY— FOSSIL CHITONS 469 



two localities. All the variations can be matched pretty well 

 in a good series of recent specimens. 



There is considerable variation in the number of insertion 

 teeth. The Point Loma shells have slits as follows : valve i, 8 ; 

 valves ii-vii, 4-3 and 3-3; valve viii, 10 to 13. Those from 

 Santa Monica have: valve i, 10 to 11; valves ii-vii, 2-2 or 

 3-3; valve viii, 10. This variation is likewise paralleled in 

 recent specimens. 



The immense northern extension of the range of this species 

 during late Pleistocene time through at least six degrees of 

 latitude from its most northern living record is noteworthy. 

 Rather oddly /. acrior appears almost always to accompany 

 /. conspicnus in the fossil state, while, except for an uncon- 

 firmed note of the latter species from Magdalena Bay, among 

 recent specimens such a coincidence has been recorded only at 

 South Bay, Cedros Island, Lower California. 15 This leads one 

 to wonder strongly whether a given species or genus may not 

 be subject to change in its ability to adapt itself to specified 

 climatic conditions through long periods of time, instead of such 

 adaptation and consequent distribution in latitude being always 

 the fixed criterion which some paleontologists seem to have 

 assumed. 



Pilsbry compares acrior more especially with magdalenensis, 

 but if our northern magdalenensis are correctly so named, it 

 seems to me that /. conspicnus is clearly the present species' 

 nearest of kin. 



Caliper measurements of the largest fossil valve seen [518a] 

 are as follows: Maximum longitude 18.3, diameter 40, altitude 

 12 mm. 



Subgenus Rhombochiton Berry, 1919 

 25. Ischnochiton (Rhombochiton) regularis (Carpenter, 1855) 

 1855. Chiton regularis Carpenter, Proc. Zool. Soc. London 



1855, p. 232. 

 1864. Lepidoplcurus regularis Carpenter, Rep. Brit. Assoc. 



Adv. Sci., 1863, p. 554, 649. 



u H. N. Lowe, — Nautilus, v. 27, p. 28. 



