128 BULLETIN 93, UNITED STATES NATIONAL MUSEUM. 



over half the occludent length. Sculpture of rounded growth ridges 

 cut into beads by much deeper, closer longitudinal grooves. Inside 

 yellowish, articular ridge rather small, reflexed, about two-thirds the 

 length of the tergal margin, abruptly truncate at the lower end. 

 Articular groove narrow. Adductor ridge well developed, nearly 

 straight, slightly overhanging the j)it of the depressor muscle. 

 Toward the basal margin it is longitudinally grooved. The occludent 

 margin is folded inward near the base. 



The tergum (pi. 32, figs. 2, 2«;) is long and narrow, width less than 

 half the length. It has a long purple beak, one-fourth the length of 

 the whole valve. The exterior is worn in the upper part, elsewhere 

 with fine cancellated sculpture of longitudinal and growth ridges. 

 The longitudinal furrow is wholly closed by infolding of the sides. 

 The basal margin slopes strongly to the spur on both sides. Spur 

 narrow, long, separated from the basiscutal angle by about its own 

 width. The spur continues on the inner face of the valve as a low 

 ridge, with which the very slightly developed articular ridge is con- 

 crescent. There is a long purple area in the middle of the inside, 

 which is elsewhere yellow. Crests for the depressor muscles are rather 

 weakly developed. The beak is penetrated by a minute pore from the 

 purple area, and its inner face has transverse arcuate ridges. 



Compartments thick and strong. Parietes strongb/ ribbed; per- 

 meated with unequal narrow tubes. Interior of parietes irregularly 

 ribbed, with short depending points in the type (having rather fine 

 ribs near the base only, in other examples). Sheath long, with deep 

 hollows under its depending edge. Radii solid, rather deeply sunken, 

 smooth, with horizontal summits when unworn; very thick, with 

 densely septate edges, the lower side of each septum denticulate. 

 Sutural edges similarly septate, the upper sides of the septa denticu- 

 late. Alas rather wide, the lateral edge of each ala hifid^ the inner 

 part acute, outer part blunt, somewhat septate. 



Basis calcareous, very thick toward the edges, and clinging very 

 firmly to the side walls. In the type-specimen there are very few, 

 irregular pores. Inside smooth except for irregularities of the sup- 

 porting rock. 



The mandible (fig. 345) has four quite small teeth, the lower one 

 adjacent to several very short, obtuse denticles. There is an ex- 

 tremely short fringe of hair on the upper margin. 



The maxilla has a slightly sigmoid edge, receding somewhat but 

 not notched below the upper pair of large spines, about four pairs 

 of smaller spines standing in the concavity. Below this the edge is 

 convex, with very large and smaller spines. There is a long fringe 

 of short hairs on the upper, and a tuft on the lower margin (fig. 34a). 



The first cirrus has wide rami, the posterior ramus three-fourths 

 as long as the anterior, with gi'eatly protruding segments, about 22 



