1898.] NATURAL SCIENCES OF PHILADELPHIA. 289 



Interior deep rose color or slightly purplish ; sutural laminae and 

 sinus about as in lignosa. Teeth rather long and somewhat rough- 

 ened, as in lignosa. Valve i with 8 slits ; ii-vii with 1-1 ; valve viii 

 entirely " Ischnoid," with regular, crescentic insertion plate, cut by 

 7 or 8 slits, which are somewhat closer posteriorly ; no sinus behind. 



Girdle leathery, nude except for solitary or two or three closely 

 grouped long bristles at all or part of the sutures, one on each side 

 of the head valve, and two behind the tail valve. 



Gills about 25 on each side, not extending quite to the anterior 

 end of the foot. 



Length 25, width 12 mm. (dried specimen), or smaller. 



In one specimen there are 9 bristles on each side and one in the 

 middle in front. Another lacks this median bristle ; and in still 

 another, some of the sutures are without bristles. In all, some of 

 them are broken off close to the girdle, leaving only stumps or pores. 



This form resembles M. simiata and imporcata in the sutural 

 bristles, but differs from them in sculpture and form of the tail valve. 

 The latter is completely as in Ischnochiton, and its form is approached 

 only by M. acuta Cpr., which however, has a slight caudal sinus, 

 much stronger sculpture, more depressed and more posterior mucro, 

 and scattered hairs on the girdle, without sutural bristles. It is 

 also blue-green or pale blue inside, while 31. Heathii is rose colored. 

 One of the specimens was dredged in 17 fms. ; station of the others 

 unknown. 



Nuttallina Thomasi n. sp. 



General form oblong, rather depressed, not keeled dorsally ; sur- 

 face granulose when not eroded. Color blackish or dark brown, 

 with a whitish band on each side of the median line of back, or irre- 

 gular whitish maculation ; girdle dark. 



Intermediate valves short and wide, with a slight or hardly notice- 

 able depression on each side of the jugum, and others in front of and 

 behind the scarcely defined, obsolete, diagonal convexity; the ante- 

 rior and posterior margins subparallel, slightly arcuate. Anterior 

 valve granulate, without radial ribs, the posterior margin excavated 

 mesially. Posterior valve with tegmentum slightly wider than the 

 anterior, the obtuse mucro somewhat behind the middle. 



Interior blue-green, with the area behind the valve-callus dark 

 brown, or livid purplish, with light sutural laminae and blue-green 

 area behind the sinus. Slits in valve i, 9 or 10; valves ii to vii, 

 1-1 ; valve viii, 10 or 11. Teeth rather separated, those of valve i 



