Vol. XI] BERRY— FOSSIL CHITONS 457 



acrior. The number of insertion teeth and slits seems to average 

 rather less than in acrior. In the fossil specimens examined 

 I find the number and arrangement of these to be generally as 

 follows: i, 9-10 (one shell has 11; that figured from Santa 

 Monica has 12) ; ii-vii, 2-2 (in one valve from Point Loma, 

 2-3) ; viii, 9 or sometimes 10 slits. 



As a fossil /. conspicuns would appear to be much less 

 abundant in most of the formations studied than its more 

 tropical congener /. acrior. 



Specimen 504, reported above with a query, may possibly 

 be an /. fnagdalenensis, as on the central areas it shows distinct 

 traces of a sculpture of the magdalenensis type, but other 

 features point toward conspicuus, and in the absence of confir- 

 matory material it seems best to leave it allocated as it is at 

 present. It is a small specimen, with a diameter of only 9.2 

 mm. 



24. Ilschnochiton (Stenoplax) acrior Carpenter, 1892 

 (Plate IX; Plate X, figs. 1-3.) 



1892. Ischnochiton (Stenoplax) acrior Carpenter, in Pilsbry, 

 Man. Conch., (1), v. 14, p. 61, pi. 14, f. 86-89. 



1917. Ischnochiton acrior Chace, Lorquinia, v. 2, p. 30 (re- 

 corded from Pleistocene of Santa Monica, Cal.). 



Diagnosis: Valves quite heavy, large, depressed to mod- 

 erately elevated, subcarinate, not beaked, all but the tail valve 

 almost straight or weakly concave behind ; lateral areas 

 strongly elevated, these and the terminal areas sculptured by 

 numerous strong, irregular, sharp, radiating riblets; central 

 areas ornamented by numerous, acute, longitudinal, wrinkly, 

 sometimes slightly anastomosing riblets ; profile of head valve 

 distinctly concave in front ; tail valve large, flattened, with low 

 mucro situated a little behind the center; sutural laminae 

 strong, deep, with a small notch at each side of the sinus ; eaves 

 wide, solid, massive, and strongly projecting past the stout, 

 sharp teeth; slits 8, 2 to 4, 13. 



Recorded Range: 



Pleistocene: Upper San Pedro Series — Long Wharf 

 Canyon, Santa Monica, California (Chare, !) ; ''Coal .Mine". 

 west side of Point Loma, San Diego County, California ( ! ). 



