166 MEMOIRS OF THE CARNEGIE MUSEUM. 



more rarely somewhat narrowed behind. Lower margin gently and rather regu- 

 larly curved, often nearly straight in the middle. Upper margin gently curved, 

 and curved down into the posterior margin, forming, -or not forming a slight angle, 

 without a distinct wing. Beaks convex, but not much swollen, only slightly 

 elevated above the hinge-hne, placed more or less anterior to the middle. Beak- 

 sculpture consisting of three to four subconcentric, fine, and rather sharp ridges, 

 the first two or three placed obliquely to the direction of the hinge-line (bent up 

 behind), the following ones showing a more or less distinct angle. On the posterior 

 slope there are several fine radiating lines, indicating additional bars, obliterated 

 on the disk. Double-looped structure of the bars is entirely absent (See beak- 

 sculpture of var. buchanensis, as figured by Marshall, 1890, figs. 13, 14). 



Shell more or less compressed to moderately swollen, greatest convexity upon 

 the posterior ridge, which is rounded and not sharply marked. Posterior slope 

 slightly compressed. 



Epidermis light green to olive, grayish, brownish or blackish, generally grayish, 

 yellowish, or light brown towards the beaks, mostly with numerous narrower or 

 wider, dark green rays, which, however, are often indistinct, and disappear in old 

 shells on account of the general darkening of the epidermis. Upon the posterior 

 slope, there may, or may not, be a few darker, broader rays. In addition, the 

 epidermis is often concentrically banded, the growth rests being marked by blackish 

 color. 



Hinge practically edentulous, straight or slightly downward curved anteriorly. 

 In front of the beaks it is more or less (often very indistinctly) incurved, and here 

 shows rudiments of cardinals in the shape of small tubercles, swellings, or mere 

 irregularities. Beak-cavity shallow. Dorsal muscle-scars faint, in front part of 

 beak-cavity. Adductor muscle-scars faint. 



Nacre bluish white, sometimes ' discolored by greenish or creamy tints toward 

 the beak-cavity; more or less iridescent, chiefly towards the posterior end. 



Shell of male and female hardly different. Although Baker (1898a, p. 73) 

 says that the shell of the female is more inflated posteriorly {i.e., at and in front 

 of posterior ridge) this inflation is not always present. If it is distinctly developed, 

 we may safely assume that we have to deal with a female, but many females do 

 not possess it. 



L. H. D. 



Size: 1. Greenville, Cat. No. 61.3294 73 mm. 38 mm. 28 mm. 



2. do. " " do 68 " 38 " 19 " 



3. Shenango, Cat. No. 61.4207 ( 9 ) 71 " 38 " 25 " 



4. do. " " do 62 " 35 " IS " 



