98 BULLETIN 93^ UNITED STATES NATIONAL MUSEUM. 



epidermis are visible near the base. The parietes have small pores 

 near the base, and the basis, in large part solid, has some rounded 

 pores near and at the periphery. 



The scutum is nearly half as wide as long, dirty white externally, 

 suffused with dark quaker drab toward the apex, with a ray of the 

 same near the tergal margin; sculpture of narrow, shallow grooves, 

 with wider flat intervals, which in the lower part bear membranous 

 borders from former opercular membranes. The articular ridge is very 

 high and oblique, abruptly truncated below, its inner face striate. An 

 acute ridge defining the lateral depressor muscle insertion runs down- 

 ward from it. Adductor ridge very strong but short, nearer to the 

 occludent than to the tergal margin, confluent above Avith the articular 

 rib. Adductor muscle insertion deep. Ai'ticular furrow deep. 



The tergum is bicolored, the carinal half deep quaker drab, scuta! 



half white. Sm'face marked with superficial grooves of growth and 



the faintest traces of radial striae. There is no furrow to the spur, but 



^^ the flat f asciole is defined by impressed lines. 



/'M^f^'^^^^ ^^^ scutal border is a little turned up. The 



/-Mlff^ '''^Sm^ spur is short, truncated distally, over one- 



r % ¥/ '^ ~-^^^m, third the width of the basal margin, and sepa- 



I h i-'-^ • --^^^M. rated from the basi-scutal angle by about 



/ ;i^iX^,^^^^^^ half its width. The articular rib is rather 



M^^ strong and runs toward the spur, leaving a 



Fig. 23.— balantjs amphitrite broad articular furrow. The scutal margin 



PEEUVIANUS, LATERAL VIEW. /..i 1*1 n • n IfTM -1 



oi the valve is broadly miiexed. ihe carmal 

 half of the interior is granose-striate. Crests for the depressor mus- 

 cles are strongly developed, terminating in little teeth projecting 

 at the basal margin. 



Compartments: The radii have strongly oblique summits and are 

 thick to the edge, which has fine, nearly straight, long, parallel, 

 weakly denticulate septa. The alee are thin, with oblique summits; 

 sutural edges finely septate, the septa very regular, minutely dentic- 

 ulate. The parietal pores are small, circular, and filled up solidly in 

 the upper part. A transverse section midway of a compartment 

 shows them partially filled, reduced to very small tubes. The inner 

 faces of the parietes below the sheath have strong, rounded, smooth 

 ribs, terminating below in minutely denticulate basal laminae. The 

 number of internal ribs is smaller than usual, 12 in a rostrum exam- 

 ined, 5 in a carinolateral compartment. The sheath is 'pur'plisTi gray, 

 with a darker lower border, strongly ridged transversely, the ridges 

 bearing minute fringes of bristles. Its lower edge overhangs shallow 

 cavities. 



Tlie basis clings very firmly to the walls, and can not be parted 

 from them without breaking the interlocking laminae. It has some 



