ART. 19. A NEW PENTAMEROID GENUS— KIRK, 7 



heights of the two along their lengths. Near the beak the septum is 

 low and the spondylium high, while in the anterior portion the rela- 

 tive height is reversed. In the intermediate region the relative 

 height shifts from one extreme to the other. The combined height 

 of the spondylium and septum is three-fourths or more the depth of 

 the combined valves. The median septum is marked by plications 

 parallel to the anterior margin. The plications may remain narrow 

 and inconspicuous or become very pronounced in the anterior por- 

 tion. Even in the latter case the line of junction of the septum with 

 the ventral valve remains a straight line. The dental plates like- 

 wise are finely striated, the striations paralleling the free margin. 



The septa of the dorsal valve are low and extend one-half to two- 

 thirds the length of the dorsal vah'-e. They gradually increase in 

 height anteriorly. In relation to one another they diverge slightly 

 from posterior to anterior extremities, but never become widely sepa- 

 rated. In the more mature portions of adult individuals the septa 

 appear practically parallel. Internally the septa stand vertical to 

 the surface of the valve or converge slightly inward. The crural 

 plates maintain about the same height throughout their length. In 

 the posterior portion of the valve the crural plates are sharply 

 reflexed so that their margins are apposed to those of the spondylium. 

 Throughout the greater part of their length the ventral margins of 

 the crural plates are approximately in contact with the dorsal mar- 

 gins of the spondylium. It follows, therefore, that anterior to the 

 region in which the crural plates are widely outspread and reflexed 

 they rapidly assume a subparallel attitude and finally become con- 

 vergent. In cross sections made some distance anterior to the beak 

 the vertical septa are found supporting rapidly converging crural 

 plates that often come practically in contact along their ventral 

 margins. This is quite different from Conchidium, in which the 

 crural plates are prevailingly divergent along their ventral margins 

 so far as known. The crural apophyses are very long, measuring 

 nearly 9.0 mm. in a specimen of average size and extending to a 

 point opposite the anterior extremity of the spondylium. The 

 apophyses are seldom well shown, as in splitting the specimens the 

 line of fracture breaks across them, as a rule. This is due to the 

 fact that the apophyses do not lie in the same planes as the septa, but 

 diverge somewhat laterally. In cross section the apophyses often 

 show where the plane of the section cuts below the level of the 

 spondylium. 



The smallest specimen of Brooksina observed, which is probably 

 referable to this species, measures 3.0 mm. in length by 3.5 mm. in 

 breadth. In this specimen the ventral valve is slightly larger and 

 more convex than the dorsal. The sinus is sharply marked on the 

 ventral valve. The plications are well developed and extend to the 



