ART. 19. A NEW PENTAMEROID GENUS — K[RK. 3 



more material available l)e found to represent abnormal types or 

 referable to some other genus. The generic diagnosis will be brief, as 

 the more detailed account of the genotype, Brooksina alaskensis, 

 new species, will be found approximately applicable to all the species 

 of the genus represented in the collections. Specific differences lie 

 chiefly in general form, character of surface sculpture, and other 

 relatively minor features. 



In Brookdna the ventral valve in side view ranges from decidedly 

 concave to slightly convex. The beak is sharply everted as a rule 

 and never involute. The beak terminates in a sharp point. A well 

 marked pseudo-area with cross striations is present. The area is 

 sharply marked off from the remainder of the valve. There is 

 usually a median sinus in the ventral valve and lacking this there is a 

 well-marked median concave or flattened area. The delthyrium is 

 broad and triangular in outline. No deltidium has been seen, but in 

 some specimens there is evidence of marginal outgrowths suggesting 

 deltidial plates. The dorsal valve is strongly convex and its apical 

 portion is sharply incurved. The surface of the valves is marked 

 by sharply defined, narrow, rounded plications. These increase in 

 number either by dichotomy, particularly in the earlier stages of 

 growth, or more commonly by intercalation in later stages. There 

 are usually well-marked concentric growth lines. The shell substance 

 is fibrous. In the anterior portion of large specimens the fibers run 

 in various directions, often at right angles to the plications. 



The internal structure of Brooksina is similar to Gonchidium. In 

 Brooksma the anterior margin of the ventral septum is straight, 

 whereas in Conchidium as known it is concave. The walls of the 

 spondjdium in Brooksina are relatively closer together than in Con- 

 chidium and converge dorsad instead of diverging, in all but their 

 posterior portions. The same is true of the crural plates. In the 

 posterior part of the valve these plates behave as in Conchidivmi. 

 Anteriorall}^, however, they tend to converge ventrad and are often 

 found touching along their ventral margins. The ventral septum 

 reaches to the anterior margin of the valve. The septum in some 

 species is sharply plicated, so that its line of contact with the surface 

 of the valve is strongly crenulate. Between the Septa of the dorsal 

 valve is a median ridge, such as is found in Pentannerus and Con- 

 oKidium. 



In the ontogeny of Brooksina alaskensis, as noted elsewhere, very 

 young individuals show a subequality of the valves, with a slight 

 preponderance of the ventral valve over the dorsal. This relation 

 changes rapidly, however, and in moderately young specimens the 

 adult characteristics are well developed. As bearing on the genetic 

 affinities of Brooksina it is of considerable interest to note that at no 



