1916] Packard: Mesozoic and Cenozoic Mactrinae 



271 



Gradational shapes connect the extremes of such a series. The tumid 

 specimens of this species also show considerable variation in the 

 juxtaposition of the umbones, ranging from a condition in which 

 they are closely approximate to that in which they are widely sepa- 

 rated in the two valves. 



The pallial sinus is a conspicuous shell character and has often 

 been considered as having a cei'tain classificatory value. As a means 

 of distinguishing Spisula catilliformis from the lutrarine species 

 Schizothaerus nuttallii the pallial sinus may be successfully em- 

 ployed ; and it is, no doubt, likewise applicable in other instances, 

 especially if the comparisons are not made between closely related 

 species. Nevertheless, Dall's 13 conclusions regarding the pallial 

 sinus for the Pelecypoda in general will also apply to the sub- 

 division of this class to which the mactrine shell belongs. Since 

 the muscle scars are also of doubtful taxonomic value they need not 

 be discussed here. 



Several different types of hinge plates are observable among the 

 Pacific Coast Mactrinae. A broad, flat one, which is more or less 

 closely associated with an elongate form of shell, is seen in Spisula 

 catilliformis (see Plate 17), S. voyi or S. falcata, while an extremely 

 narrow hinge plate is seen in Mantra exoleta or S. selbyensis. The 

 shape of the chondophore is modified to a certain extent by the 

 dynamic forces which determine the general form of the shell. The 

 elongate specimens of Mulinia densata or Schizothaerus nuttallii pos- 

 sess relatively shallower and broader resilifers than do the shorter in- 

 dividuals of the same species. This suggests a possible difficulty in 

 the way of using the hinge for the basis of classification. A similar 

 response to the changing shape of a species might be conceived as 

 applying to the entire dentition and thereby depreciating the value of 

 the taxonomic system advocated in this paper. In so far as there is 

 a variation in shape involving the dorsal portion of the shell, just so 

 far must there, of necessity, be an accommodation of the dental arma- 

 ture to these changing conditions. In the case of an anterioposterior 

 compression of a shell, as may be seen in the ventricose forms of 

 Midinia densata, a corresponding crowding of the dental units is 

 inevitable ; but regardless of this concentration of the elements there 

 still remain certain fundamental differences, for this crowding in- 

 volves all of the elements of the hinge in such a way that each tooth 



Dall, W. H., Am. Jour. Sci. (3), vol. 38, p. 446, 1889. 



