CLIXTOX, LOWER IIELDEUUERG, CllEMUNC;, AND WAVEIJLY GUOUPS. 4."]!) 



and abruptly towards the hinge line. At the cardinal extremities there is a flattened 

 area 2 mm. or more in width, gradually growing nari-ower to the beak. 



Dorsal valve, variably concave, sometimes nearly flat, at other times following the 

 contour of the ventral v:ilvo; always with a flat area corresponding to that of the 

 opposite valve. 



Surface marked by rounded or subangular striiv, which increase by bifurcation ; 

 at the base there are four in tlxe s])ace of 1 mm. The radiating stria' ai-e crossed by 

 extremely fine concentiic striie, about twelve in the space of 1 mm., and at irregular 

 intervals by more distinct lines of growth. 



On ihe hinge line are evidences of eight spines, four on each side of the beak, 

 short, slightly oblique. 



The inteiior of the ventral valve shows a narrow median ridge, with narrow oval 

 occulsor muscular impressions. Corresponding to the radiating striae of the exterior 

 are rows of small conical nodes, the bases of which are nearly in contact. They con- 

 tinue nearly half way to the beak. 



The impression of the interior of the ventral valve has numerous punctai or 

 holes, caused by the mucronate nodes of the interior ; it presents a peculiar clathrate 

 appearance, which is sutticient to distinguish it from any other species. 



Fonnalion and locality. Lower Heldcrberg gronp, Mansing's quariy, near Ilaz- 

 ardville, Carbon county, Pennsylvania. 



CYRTINA TRIPLICATA Simpson, n.sp., Fig. 4. 



Shell more or less triangular, subpyramidal ; hinge line equal to the greatest 

 width of the shell ; proportional length and width somewhat variable ; length of the 



4 



r 





iC.PS ' Yr.l083. 



dorsal valve from one-half to two-thirds the width ; length of ventral valve about 

 equal to the width ; height of the area greater than the length of the dorsal valve. 



Ventral valve quadrilateral, subpyramidal ; most i)romiuent at the beak, which 

 is variable in elevation, straight, not arching over the area, usually attenuate ; lateral 

 margins regularly rounded. Mesial sinus narrow at the beak, rapidly widening as it 

 extends forward, and becoming deep ; where it reaches the margin the shell is pro- 



