DIAPHRAGMUS 137 



umbonal region to the anterior margin, the mesial sinus obsolete or very 

 shallow and ill-defined ; the beak strongly incurved. Surface of the valve 

 marked by rounded, radiating costae which increase usually by bifurca- 

 tion in the posterior portion of the valve, and continue to the front margin 

 with much less frequent divisions, in some specimens becoming more or 

 less fasciculate towards the front, at the front of mature shells the costae 

 are usually separated about one millimeter or less from center to center, 

 becoming somewhat smaller upon the lateral slopes towards the cardinal 

 extremities ; crossing the radiating plications upon the posterior portion 

 of the valve are rather inconspicuous, concentric, wrinkle-like markings 

 which are often nearly obsolete, other exceedingly fine concentric mark- 

 ings cover the entire surface of the valve when it is well preserved ; spine 

 bases are rare or nearly obsolete upon the greater portion of the surface 

 of the valve, those present being sparsely scattered, usually towards the 

 front, but upon the auriculations and the lateral slopes just inside the 

 auriculations, is a group of rather fine, crowded spine bases, sometimes 

 forty or more in number. 



Brachial valve gently concave or flattened in the visceral region, 

 curving more strongly anteriorly and laterally where the outer margin 

 of the valve is conspicuously produced nearly in contact with the inner 

 surface of the opposite valve, not infrequently the inner portion of the 

 gently concave visceral part of the valve is more or less differentiated, 

 the differentiated region being somewhat more concave and being bound- 

 ed by a rather strong, concentric line of growth. Surface of the valve 

 marked by radiating costae similar to those of the opposite valve, often 

 being somewhat better defined posteriorly in the more concave differ- 

 entiated portion of the valve; ill-defined, concentric, wrinkle-like mark- 

 ings, usually stronger than those of the opposite valve, occupy the visceral 

 portion of the valve and the entire surface is covered by very fine con- 

 centric markings similar to those of the opposite valve; spine bases 

 commonly if not always absent. Internally the posterior visceral portion 

 of the valve is sharply differentiated from the anterior and antero- 

 lateral produced portion of the valve, by the development of a partition 

 which is apparently a continuation of the gently concave or flattened 

 visceral portion of the valve ; not infrequently the specimens break along 

 this partition separating the produced portion of the shell entirely from 

 the visceral portion and giving it quite a different aspect from the entire 

 shell. Other internal characters of the shell not observed. 



Remarks. This species is easily recognized on account of the sub- 

 quadrangular form of the posterior view of the shell due to the broadly 

 flattened mesial portion of the pedicle valve and the nearly vertical posi- 

 tion of the lateral slopes towards the cardinal margin. Other characteris- 

 tic features of the species are the short hinge-line, the crowded group 



