28 NEW YORK STATE MUSEUM 



Pterinea (Pteronitella ?) incurvata Clarke 



Plate 3, figures 13-18 



Pterinea (Pteronitella?) incurvata Clarke. N. Y. State Mus. Bui. 107. 1907. 

 p. 210 



Valves elongate on the hinge, the greatest length of the hinge being 

 almost twice the hight of the shell. Anterior wing well defined on both 

 valves, byssal sinus not deep but broad and not marked by a notch on the 

 right valve. Beaks one third the length of the hinge from the anterior 

 extremity. General outline very oblique. Left valve highly convex and 

 incurved over the body, sloping abruptly to the posterior wing, more grad- 

 ually to the broad byssal sinus in front. From the prominent umbo the 

 crescence line swings in a curve backward and forms a strong projection on 

 the lower margin. The posterior wing is extended well beyond the poste- 

 rior margin of the body and bounded by a concave curve which terminates 

 in an acute point. Its surface is depressed in a direction conforming with 

 the curve of the body. The surface of this valve is covered with regular 

 concentric growth lines which are essentially unmodified on the anterior and 

 posterior wings but the body of the valve bears radial striae which have some- 

 what the aspect of unequal and flat riblets produced by series of incised 

 lines. These multiply and broaden unequally presenting much the same 

 aspect as those in P . e d m u n d i of the Chapman Plantation. [See p. 103] 



The right valve is depressed ; on the posterior wing deeply concave, 

 convex but not elevated along the crescence line, thence sloping to the lower 

 margin with an incurved surface, the postlateral edge of the valve being 

 upturned. The byssal notch and sinus are indicated by a marginal incur- 

 vature and depression. One specimen shows the striated ligament area, a 

 small anterior adductor and slender anterior tooth. Surface of this valve 

 entirely smooth or with concentric lines only. 



This shell is characterized by its extreme convexity and incurvature. 



Horizon. No. 11. 



Pterinea brisa var. vexillum Clarke 



Plate ;, figures 5, 6 



Pterinea brisa var. vexillum Clarke. N. Y. State Mus. Bui. 107. 1907. p. 209 



A left valve is suberect with a semicircular lower margin, deep byssal 

 sinus, short but well defined anterior wing and broad posterior wing extended 

 to an acute posterior angle. Its surface is flat or slightly concave in the 

 pallial region. The sculpture consists of fine radial riblets of subequal size 

 moderately distant and numerous over the body of the valve, very obscure 

 on the posterior wing, which is entirely covered by concentric crowded 

 lamellose lines; the latter are extremely faint over the rest of the shell. A 



