MOPALIA. 299 



tured with plume-like radii, granose at the sutures, the diagonal ribs 

 and the radiating riblets of the head-valve ; the grains elongated, 

 subspinose, elegantly serrating the sutures. 



Interior pale blue. Tail valve having the posterior sinus hardly 

 indicated externally, small, with a minute tooth set in at the apex; 

 at one side it has 2, at the other 3 slits ; slit-rays inconspicuous. 

 Central valves having 1, anterior valve 8 slits, the slit-rays con- 

 spicuous; teeth acute, small. Eaves small, very spongy. Sinus 

 small, deep, flat, the sutural plates separated. 



Girdle thin, narrow, smooth, sparsely and irregularly beset with 

 subtranslucent, larger and smaller tubular bristles. 



Length 16i, width 11 mill. ; divergence 112. 



Habitat unknown. 



The above description, from Carpenter's MS., was drawn from a 

 single specimen in the collection of Dr. Wesley Newcomb (now of 

 Cornell College). The figures 80, 81 represent the same specimen. 



Var. fissa Cpr. Sculpture between that of M. acuta and M. plu- 

 mosa ; the granules at the suture and diagonal rib hardly claw- 

 shaped ; color and form also intermediate. Interior : posterior 

 valve having two slits on one side, one on the other, the median 

 caudal sinus minute, acutely slit ; otherwise normal. Girdle smooth 

 with sparse, small spines. 



Length 15, breath 10 mill. ; divergence 115. 



This form was described by Carpenter from a single specimen 

 from Monterey. It differs from acuta and plumosa in lacking the 

 " key-stone " like structure of the caudal sinus ; but the series 

 examined by me proves that this character has no constancy. 



Subspecies M. LIGNOSA Gould. PI. 63, figs. 58, 59 (typical) ; 60, 

 61, 62, 63. 



Shell oval, elevated, carinated or angular at the dorsal ridge, the 

 side-slopes straight. Surface lusterless, apparently smooth ; grayish, 

 greenish or bluish with radiating streaks, lines and flammules of 

 brown or purple-brown. 



Under a lens the lateral areas appear nearly smooth ; the central 

 areas being closely and finely pitted all over. Girdle narrow, 

 sparsely hairy. 



Interior white and light blue. Length 60, breadth 35 mill.; 

 divergence 120. 



Vancouver I. to Monterey, California. 



