Vol. XI] 



BERRY— FOSSIL CHI TOMS 



487 



TAIL VALVE 



Flattened above. 



Mucro strongly posterior (less so 

 in juvenile specimens). Posterior 

 area with 5-6 heavy ribs, at first 

 simple, but developing from 1 to 

 4 grooves toward the base. 



Slits 14-18. 



Longitudinal ribs of central areas 

 5-8 on a side. 



Mucro strongly anterior, the large 

 posterior area usually immensely 

 thickened and elevated behind the 

 mucro; ribs 5-7, heavy, and though 

 at first simple, soon developing a 

 strong median furrow, with some- 

 times 1 or 2 secondary furrows to- 

 ward the base. 



Slits 18-25. 



Longitudinal ribs of central areas 

 7-9 on a side. 



The marginal toothing in the anterior valve of this species 

 is peculiar and apparently characteristic, the remarkable fea- 

 ture being that certain of the teeth are excessively minute clue 

 to the close approximation in pairs of the slits which bound 

 them, a pair of slits in such cases being brought into coincidence 

 with one of the external ribs instead of the usual single slit. 

 The typical arrangement, where the duplication affects only 

 the slits lying immediately on each side of the central one is 

 clearly shown in Pilsbry's figure in the "Manual" (op. cit.. p1. 

 58, fig. 4), but this has not become so fixed but that it is subject 

 to some strangely irregular variation. 21 The gross number 

 of slits is ordinarily 9, but even this is subject to modification. 

 Reducing the number and arrangement of the slits with respect 

 to the ribs to a formula, 17 Santa Monica specimens examined 

 yield the following figures : 



total 9 slits 

 " 10 " 

 7 *' 



total 10 slits 



11 



Arrangement symmetrical — 



11 show the formula 1-1-2-1-2-1-1, 

 2 " " " 1-1-2-2-2-1-1, 



1 shows " " 1-1-1-1-1-1-1, 



Arrangement asymmetrical — 



2 show the formula 1-1-2-1-2-2-1, 

 1 shows " " 1-2-1-1-1-1-1, 

 1 " " " 1-2-2-2-2-1-1, 



With one single exception (which is 8-ribbed), all the above 

 specimens have 7 strong primary ribs, simple at their origin, but 

 soon developing a median sulcus and eventually in well grown 

 specimens showing a very constant tendency to the formation 

 of an accessory sulcus on each side of the primary one toward 

 the base. 



« Of course it is possible that if a sufficiently large series of specimens could be ex- 

 amined, the variations would not appear so irregular. 



