56 



BULLETIN 93, UNITED STATES NATIONAL MUSEUM. 



ridged, tlie ridges near the base slightly crenulated by longitudinal 

 strise, which are very fine or subobsolete in the intervals. The 

 articular ridge is high, flattened and reflexed, not much more than 

 half the length of the tergal margin, its lower end usually overhanging. 

 Adductor ridge confluent with the articular above, continued down- 

 wards in a broad, rounded callous ridge or merely a general thicken- 

 ing of the median part of the valve, bounding the depressor muscle 

 scar. Muscle impressions deep; occludent edge folded over near 

 the base. 



The tergurn (pi. 10, figs, la, 1&) is purplish toward the apex, 

 triangular, broad, delicately ridged, with the groove to the spur 

 closed. The spur is long, and separated by about twice its width from 

 tlie lasiscutal angle. The articular ridge and the reflection of the 

 scutal edge are both high and acute. The external ridges denticulate 

 the scutal border. 



FlG. 9. BALANU3 TINTINNABULUM. a, MIDI>LE SEGMENT OF CIRRUS VI. 6, MANDIBLE. C, MAXILLA. 



d, LAB RUM. 



Compartments. The radii are wide, with level summits. The outer 

 lamina of the wall bears numerous short lamellae on its inner edge 

 (pi. 10, fig. le) between the septa. The parietal tubes are entirely 

 open, except quite near the summits, where they have numerous 

 transverse septa. The interlocking septa of the radii and sutural 

 edges are straight, regular, and denticulate on both sides. 



The labrurn has a straight, shortly hairy edge and narrow notch. 

 In the specimen examined there are two small teeth on one side, one 

 on the other (fig. 9cZ). 



The mandible has four strong teeth, the fifth tooth small, united 

 with the blunt lower point (fig. 96). 



The maxilla has a straight edge and 14 large spines; those below 

 the upper large pair are in two series (fig. 9c). 



The cirri i to iii are shorter than in other groups of the genus. 

 Cirrus i has rami about 5 mm. long, of 17 and 15 segments, those of 



