318 



BULLETIN 93, UNITED STATES NATIONAL MUSEUM. 



The opercular valves are peculiar from the conspicuous disparity 

 in size and shape between the right and left valves. In a very great 

 majority of the specimens examined the left valves are larger than 

 the right and assume the position of the free valves of Vernica. 

 Plate 74, figures 2 to 2fZ, are drawn to the same scale, and repre- 

 sent valves of one individual. 



The larger scutum has nearly the shape of an isosceles triangle. 

 The exterior has conspicuous, not very close laminae of growth and 

 a shallow radial sulcus which divides off the tergal third. The articu- 

 lar ridge is well developed, with a rounded outline. Articular fur- 

 row rather deep. The adductor muscle impression is small and 

 not very deep. No adductor ridge. Pit for the lateral depressor 

 muscle is crossed by delicate crests. 



Fig. 93.- 



-Chthamalus anisopoma. a, maxilla, to, L.vcnuM. c, inteumediate segment 

 OF CIRRUS VI. d, Ciithamalus dalli, labuum. 



The smaller scutum is long and very narrow, the tergal margin 

 much less than half of the basal. Articular rib well developed, long. 

 The deep pit for the lateral depressor muscle is upon the basal edge 

 of the valve. 



The larger tergum is irregularly oblong. Articular ridge angular, 

 strongly produced over the scutal margin. Spur very short and 

 rounded. Basal margin straight. Crests for the depressor muscle 

 usually developed strongly. 



Smaller tergum triangular, shaped somewhat like tliat of Chtham- 

 dlus fssus^ but broader; basal margin somewhat sigmoid; articular 

 ridge small. 



The labrum has a very broadly V-shaped edge, set Avith teeth, and 

 many short hairs (fig. 937>). 



The mandible is like that of Naples C. stellatus. 



The maxilla (fig. 93«) also resembles that of C. stellatus. 



Cirrus i has G and 5 segments. 



Cirrus ii has 5 and 7 or 6 and 8 segments, the i^osterior shorter by 

 two segments. 



Cirrus vi has rami of 20 segments, bearing five pairs of spines 

 (fig. 93c). Both cirri and spines are quite slender. 



