MARYLAND GEOLOGICAL SURVEY 707 



latter are smooth or finely granular, while those of the former are, as a 

 rule, transversely striated. 



The genus extends well back into the Cretaceous, and, though not 

 abundantly represented in the recent seas, its occurrence is almost uni- 

 versal. 



Subgenus CYMBOPHORA Gabb 1 



Type. Mactra ashburneri Gabb. 



"The hinge is composed of a rather heavy hinge plate, bearing a 

 cartilage-pit, not sunk into its substance, as in the others of the Mactridce, 

 but, as it were, built up on its surface; a small, delicate, spoon-shaped 

 process, laid obliquely under the beaks, its base being on, or slightly 

 above the level of the hinge plate ; in the right valve the cardinal tooth is 

 single, very delicate, and nearly at a right angle with the anterior wall of 

 the cartilage-pit ; in the left valve the tooth is V-shaped, entirely separated 

 from the pit, very slender, and articulated between the tooth and the pit 

 of the opposite side; the lateral teeth are large and comparatively very 

 robust." Gabb, 1869. 



" A careful study of the typical species of this group shows that it differs 

 from Spisula only in the following features: The attached ends of the 

 resilium were convex instead of flat (as is sometimes seen in recent 

 species), and the margins of the pit are therefore elevated; while the pos- 

 terior sinus, instead of being (as usually in the later types of Spisula} 

 roofed over or filled up with a solid mass of callus at the apex, upon which 

 the ligament is attached, is vacant, so that the ligament was fixed on the 

 convex margin of the pit, or on the side of the ventral lamina, or partly 

 on both, all being very close together. This character would seem to be 

 trifling until it is observed that all the Mesozoic species are characterized 

 by this feature, though, as in recent Spisula, the external form may vary, 

 the dorsal areas be smooth or grooved, the teeth sulcate or smooth. As it 

 is common to all the Cretaceous Mactridce of which I have been able to 

 examine a hinge, I have thought it best to retain the name in a subgeneric 

 sense for that stage of development of the group." Dall, 1898. 2 



1 Geol. Survey of California, 1869, Pal., vol. ii, p. 180. 



2 Trans. Wagner Free Inst. Sci., Phila., vol. iii, pt. iv, p. 879. 



