1916 J Packard: Mesozoic and Cenozoic Mactrinae 267 



of shell. In Mulinia the union of these elements is even more inti- 

 mate, since they are completely submerged in a single well-defined 

 pit. 



In all of these genera there is a process known as the spur, 

 which arises from the dorsal shell margin just posterior to the 

 , cardinal, and extends out over the chondophore. In the mactroid 

 species this is often fused to the partitioning ridge above mentioned 

 in such a way as to separate the ligament distinctly from the 

 resilifer. In other cases the spur coalesces with the dorsal margin 

 of the shell, forming a roof over the upper end of the pit for the 

 resilium. 



Three areas adjacent to the cardinals have been named by Dall 12 

 the anterior, the posterior and the ventral shmses. The anterior 

 sinus is situated between the anterior arm of the cardinal and the 

 dorsal margin of the shell. The posterior one bears a similar relation 

 to the posterior arm. The area between the cardinal arms and the 

 ventral margin of the hinge plate is known as the ventral sinus. 



VAKIATIONS OF THE ELEMENTS OF THE MACTRINE 



SHELL 



The various elements of the shell that might have a taxonomie 

 value would include : the general shape, the position of the beaks, 

 the type of the pallial line with its sinus, the muscle scars, and the 

 dental armature. The utility of these characters will depend upon 

 their specific constancy. It is therefore important for the purposes 

 of this paper to discuss each of these details of the mactrine shell. 



The members of this subfamily are found to vary specifically 

 from an equivalve and nearly equilateral form such as Mactra 

 californica to the inequilateral species Spisula hemphiUii, and from 

 the slightly convex form of the latter to that of the very ventricose 

 species Mactra exoleta. The outlines of the mactrine shell vary from 

 a trigonal to an elongate type, which may be illustrated respectively 

 by the species Spisida merriami and S. falcata. Of even more 

 immediate interest are those variations that occur within a single 

 species. 



If a number of mactrine forms are studied, it is found that there 

 is a marked degree of variation of the general shape within the 



12 Op. cit., p. 864. 



