6 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.60. 



a sharply incised sinus its place is taken by a well-defined flattened or 

 slightly concave area. As a rule the sinus is most clearly marked 

 near the beak. Toward the anterior margin it widens rapidly and 

 shows as a broad shallow depression. 



The cardinal area of the ventral valve is broad and high. The 

 striations on the area parallel the valve margins and meet the 

 margins of the delthyrium at high angles. They do not radiate from 

 the beak as do the plications of the valves. The area apparently 

 can be classed as a cross striated cardinal area of a nontypical sort. 

 The hinge line is not straight but sinuous, therein differing from the 

 characteristic true cardinal areas of typical forms. On this account 

 it might be better to call an area of this sort a pseudo-area rather 

 than a true cardinal area. The delthyrium is broad and apparently 

 lacks a deltidium. There is some evidence of the presence of margi- 

 nal outgrowths resembling deltidial plates, but the structure is not 

 clear. 



The apical portion of the dorsal valve is sharply incurved and 

 partially fills the delthyrium of the ventral valve. The dorsal valve 

 is most highly arched in the posterior portion. In some specimens 

 there is evidence of a slight median flattening, corresponding to the 

 sinus of the ventral valve, but in most specimens this is wanting. 



The valves are traversed by numerous fine, sharply marked, 

 rounded plications. As a rule these multiply bj^ dichotomy, but in 

 older individuals, especially near the anterior margin, the number 

 is frequently increased by intercalation. The plications are equally 

 pronounced on both valves. Concentric growth lines are well 

 marked, and between these the plications are crossed by minor lines 

 that break up the individual plication into a series of beadlike 

 nodes. This character is more pronounced in some individuals than 

 others, and is specially well marked in large specimens near the 

 anterior margin. The growth lines are continuous with those in 

 the pseudo-area. 



In Brooksina alaskensis the median septum of the pedicle valve as 

 a rule reaches quite to the anterior margin of the valve. Its anterior 

 margin is straight. The median septum from its point of inception 

 at the beak gradually grows higher anteriorly, reaching its maxi- 

 mum height at the anterior margin. The spondylium is very broad 

 and deep in the posterior portion. It narrows very rapidly ante- 

 riorly and loses its height more gradually in the same direction. 

 In its anterior portion the walls of the spondylium are but slightly 

 separated and in cross section cases have been seen where the dorsal 

 margins of the plates come in contact. The ventral extremity of the 

 spondylium projects but slightly beyond the anterior margin of the 

 ventral septum. As the median septum and spondylium increase in 

 height in op]:)osite directions there is great diversitv in tlie relative 



