A. E. Verrill — MoUufica of the New England Coast. 207 



slight, rounded, median notch ; the surface is marked with faint 

 radiating grooves, and is uniformly covered with small rounded 

 granules. The succeeding valves have their posterior border nearly 

 straight, with a slightly projecting, obtuse, median beak, from which 

 run well-marked, elevated, rounded diagonal ribs; the lateral areas 

 are somewhat raised, with a depression next the ribs, and their sur- 

 face is covered with small, low rounded granules, more distinct than 

 those on the median areas, which appear nearly smooth to the naked 

 eye, but are crossed by evident transverse lines of growth. The 

 posterior valve has the posterior edge a little upturned, and slightly 

 emarginate in the middle, with a submarginal, raised rib near the 

 posterior margin above ; the upper edge overhangs the lower 

 lamina but slightly, or not at all, in the gmaller specimens, with a 

 deep groove between ; the lower lamina is more deeply emarginate, 

 in the middle, than the upper one, with wider laminae each side of 

 the notch, but in the largest example the upper portion is divided 

 into several laminae, and projects decidedly beyond the lower, while 

 the notch is obsolete, (Perhaps this is due to injury during life.) 



When detached, the inserted edges of the valves are very narrow, 

 the front edge of the anterior valve is narrower than the upper, 

 lamina thickened and divided into numerous (about thirty) small, 

 rough and unequal denticles, which become obsolete near the lateral 

 angles. The median ])lates have the lateral insertion plates small, 

 truncate, not i)rojecting beyond the uppei' lamina, with a well- 

 marked groove between, and separated from the anterior insertion 

 plates by a deep narrow notch, in line with the diagonal ribs ; the 

 anterior plates are broadly rounded, not very wide, separated by a 

 rather wide rounded median notch. 



Color of marginal membrane dull rusty or yellowish brown ; shell 

 grayish white, stained with brown. 



Length of the largest specimen, in alcohol, 32""" ; its greatest 

 breadth, 26'"™ ; length of shell, 21'"™ ; greatest breadth, 18""" ; length 

 of anterior valve, 4"""; its bi-eadtli, 15-5"'"'; length of exposed part 

 of 3d valve, 4"'"^ ; of posterior valve, 4*7""" ; its breadth, 8"'™ ; ex- 

 tent of mai'ginal membrane beyond the shell, anteriorly, 12""" ; 

 length of foot, 12'""'; breadth of foot, 8™"'; length of head, 3"""; 

 breadth, 7""'". 



A sjiiall specimen is 21""" long; breadth, 16'""'; expanse of mem 

 branein front of shell, 6-5"""; length of shell, 16'""' ; its breadth, 13-5"'"' 



When living the marginal membrane was relatively broader. 



