-PJC 



the ribs imo -.tel, but at their disbnl ends irregularly spaced, 



arched incremental lines usually; visible; left valve with the sa-iie 

 number o'^ ribs, b^it lovrer, .,._e rounded and -nf.h q t n -i^ 1 v. o- si'les dis- 

 tally, the interspaces \7ider; the concentric lines in the inter- 

 s^^aces snd the growth lines on Hie ribs more prominent than on the 

 Tigh^' v-ivp; r:'ubma.rgins on both valves smooth, exoe"ot for ver- 

 fine GOiicentric lines; auricles subequal, the po sterior vj'ith two 

 to five small radial ribs, decreasing- in prominence from the mar- 

 gin of the di sir or absent on the dorsal -oortion, ':he entire sur- 

 face covered with fine concentric lines; anterior auricle seulp- 

 txired with three to five usually slightly coarser ribc of irregiil- 

 ar vildth, slightly rourhened by the over-riding, inconspicuous con- 

 centric lines; cardinal crura prominent; interior ventral margin 

 deeply crenulated, with lira'"' extending toward the imbo. 



Dimensions ,- A right Valve: lat., 25.6 m"'.; pit., 5^5.5 mm.; 

 diam. , 8 mm.; length of hirjge, 13 mm. A left valve: lat., 26.5 

 mm.; alt., 25.8 ram,; diam., 5.S mm.; length of hinge, 14 mm. 



HeriB, r^-^s .- ' This form is by far the most ab^^' -■''^■^t of the Borden 

 Pectens. ^lae right valve is much inflated, whi" ■ left is less 



convex or even compressi , 3 eeially among young indiviiiuals. 

 ■''he degree of inflation of both valves is subject to slight vari- 

 ation; the most inflated forms are also less orbicular, being high- 

 er than wide. The difference in the character of the radial sculp- 



