266 



University of California Publications in Geology [Vol. 9 



articles upon this family published in the Proceedings of the 

 Malacological Society of London, 10 and in the Transacticms of the 

 Wagner Free Institute of Science. 11 



The mactroid hinge consists of a A -shaped cardinal tooth, situ- 

 ated just anterior to the resilifer, and an anterior and a posterior 

 lateral in the left valve, which fit between two laterals, or laminae, 

 in the right valve. The cardinal of the left valve is formed of two 

 parts, an anterior and a posterior arm. These arise from the hinge 

 plate and are fused along a dorsal median line. The posterior 

 arm often supports, at its base, a secondary process, which 

 may overhang the pit or chondophore, and may extend parallel to 

 the arm of the cardinal to a height equal to that of the cardinal 

 itself. This accessory lamella is strongly developed in the lutrarine 

 species Schizothaerus nuttallii (Conrad). This may be clearly seen 

 in the figure of that species found on plate 35. Other lamellae occur 

 upon the laterals of certain species, notably upon those of Mactra 

 calif ornica. The cardinal of the right valve differs considerably 

 from that of the left. The more diverging arms of this tooth fit 

 outside those of the left cardinal of the opposite valve. In the 

 right cardinal the arms are often unequally developed. In the case 

 of those species belonging to the subgenus Cymbophora the posterior 

 arm is entirely absent, while other subgenera are characterized by 

 having the cardinal arms of nearly equal development. Mactra 

 exoleta is a good example of such a form. Furthermore the 

 cardinal arms are seldom united along a median line as is the case 

 in the left valve, but are slightly separated and joined to the dorsal 

 margin of the shell. 



The resilium and the ligament bear a varied relation to each 

 other and serve, therefore, as important bases for generic definitions. 

 In the genus Mactra the two are separated by a shelly ridge which 

 extends ventrally from the beak. This fragile partition is but im- 

 perfectly seen in many of the typical mactroid species. It is repre- 

 sented in the left valve of Mactra exoleta by a low ridge extending 

 from a point under the beaks to the ventral-posterior margin of the 

 resilifer. A condition that is somewhat comparable is seen in the 

 lutrarine species Schizothaerus nuttallii. In the genus Spisula the 

 resilium and the ligament are not separated by any limiting wall 



10 Dall, W. H., Proc. Mai. Soc. London, vol. 1, pp. 203-213, 1895. 



11 Dall, W. H., Trans. Wagner Institute Sci., vol. 3, pt. 4, pp 862-891, 1898. 



