508 



MEMOIRS OF THE CARNEGIE MUSEUM. 



Old specimens are generally uniformly black; younger ones are dark greenish 

 olive or brownish olive, sometimes with more or less distinct concentric lighter 

 (brownish to yellowish) bands. 



Hinge-line gently or more stronglj- convex. Ligamental sinus over the pos- 

 terior part of the lateral teeth; in larger specimens over the last fifth. Laterals 

 rather straight in young, curved in older shells, one in right, two in left valve, 

 their edges somewhat rough. Pscudocardinals normally' two in right, and one in 

 left valve, but often there is a second (posterior) one in the left valve. These 

 teeth are extremely variable. In younger shells they are compressed and lamellar, 

 directed obliquely forwards; the posterior tooth of the right valve is always thicker 

 and higher than the anterior, and they are, chiefly the former, crenulated or denti- 

 culated. In older shells, the posterior tooth of the right, and the (anterior) tooth 

 of the left valve may become thicker, more stumpj', and may be much divided. 

 The posterior tooth of the left valve, if present at all, may be larger or smaller, 

 compressed or stumpy. 



Cavit.y of shell and beaks moderate. Nacre in all my specimens white, iri- 

 descent posteriorly. Anterior adductor-scar distinct and impressed; anterior 

 retractor-scar separated from it, small and deep; anterior protractor-scar connected 

 with it. Posterior adductor-scar less impressed, the posterior retractor-scar forming 

 an upper process of it. Pallial line distinct. Dorsal muscle-scars a few, situated 

 in the beak-cavity. 



Measurements. 



Length. 



Height. 



Diameter. 



Beaks. 



34 



48 

 54 

 54 

 58 

 61 

 62 



20.5 mm. =60 pr. ct. of L. 



26.5 



35 



30.5 



31 



35 



35 



=55 

 = 65 

 = 56 

 = 53 

 =57 

 =56 



12 mm 



20 " 



27 " 



24 " 



26 " 



26 " 



27 " 



. =35 pr. et. of L. 

 =41 . " 

 =50 

 = 44 

 =45 

 =42 

 =44 



at 10 

 II) 

 14 

 13 

 13 

 17.5 

 15 



mm. =29 pr. ct. of L. 

 '• =21 



= 24 

 = 22 

 =29 

 =24 



For comparison I give here previous measurements. 



