272 University of California Publications in Geology [Vol. 9 



still retains its relative position with reference to that of the others. 



Dall 14 has shown that the mactrine cardinal tooth revolves, as 

 it were, upon an axis in such a manner that the cardinal arms 

 occupy different positions upon the hinge plate in the different 

 species. This apparent rotation is seen most clearly in the right 

 valve. Spisula catilliformis and S. planulata serve as extreme ex- 

 amples of this condition. Although this shifting of the position of 

 the cardinal arms is noticeable when different groups are studied, 

 it is negligible when species are considered among themselves. In 

 certain forms there is a marked tendency for the arms of the left 

 cardinal to thicken in such a way as to form a massive triangular 

 block-tooth instead of the typical mactrine cardinal composed of two 

 distinct arms. This condition appears to be common in the heavy, 

 thick-shelled forms like Mulinia densata. A similar tendency is seen 

 in Mulinia alta, where a thickening of the cardinal arms occurs, 

 although the hinge teeth are much more delicate than in the form 

 just mentioned. The shorter specimens of Schizothaerus nuttallii 

 appear to have the same type of cardinals. This may be attributed 

 in part to senility, but not entirely so, for the younger specimens 

 show the same tendencies to a lesser degree. The relative height of 

 the cardinal is variable. It is perhaps less noticeably variable in 

 the mactroid species than in the spisuloid ones. 



In discussing the laterals Dall 15 writes : ' ' the paired laminae are 

 more variable than the single ones". There is, then, a recognized 

 variation of the laterals as well as of the cardinals. This variation 

 may be expressed in terms of length, height and thickness of the 

 teeth. The lamella on the arm of the left cardinal is so fragile that 

 it is seldom seen on fossil specimens, and only occasionally in Recent 

 material. It is quite variable in most of the mactrine species, judg- 

 ing from the few specimens in which it has been seen and from its 

 known inconstancy in the indigenous lutrarine species. 



The question now arises regarding the status of a proposed 

 classification based primarily upon a hinge which is as inconstant as 

 has been , indicated above. Dall 16 says : ' ' The distribution of the 

 parts of the hinge in a single species is remarkably uniform, but if 

 groups of species are considered, the types seem to gradually 

 approach and almost merge one in another." After making this the 



14 Dall, W. H., Trans. Wagner Inst. Sci., vol. 3, pt. 4, p. 869, 1898. 

 is Op. cit., p. 868. 16 Op. cit., p. 867. 



