184 FIFTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



prominent striae, which are both bifurcated and implanted, often appearing fa- 

 sciculate near the margin of the shell. 

 The largest specimens known of this species measure not more than three-eighths 



of an inch in the greatest diameter. The distinguishing features are the coarse 



prominent striae and the length of the cardinal line. 



Geological formation and locality. Shales of the Hamilton group : Western 



New-York. 



ORTHIS PENELOPE ( n. s.). 



Shell large, oblate, the proportions of length and breadth usually as four to five, 

 plano-convex : hinge-line about two-fifths of the breadth of the shell. Ventral 

 valve flat or slightly convex : beak somewhat elevated; foramen broad, triangu- 

 lar. Dorsal valve regularly convex, with a very slight depression : beak small, 

 rising but little beyond the general outline of the shell; area smaller than that 

 of the opposite valve. 

 Surface marked by fine radiating bifurcating strige, which are strongly arched 

 upwards near the cardinal extremities, and crossed by fine concentric striae, 

 giving a slightly rugose appearance in well-preserved specimens; and, besides 

 these, are closely arranged lamellose lines of growth. The radiating striae have 

 the appearance of being broken, from the peculiar manner in which the pores 

 open upon the surface. 

 Interior of the ventral valve marked by a subcircular foliate muscular impression, 

 which occupies more than half the length and breadth of the valve, and, in old 

 specimens, is extremely thickened from its anterior margin nearly to the border 

 of the palleal impression. Interior of the dorsal valve marked, in old specimens, 

 by a similar imprint, but smaller and less distinctly defined. The cardinal and 

 brachial processes are strong and prominent, directed downwards into the op- 

 posite valve : the cardinal process fills the broad foramen of the ventral valve, 

 and appears as an angular ridge on the exterior of the area. 

 This species is much larger than O. vanuxemiy with which it is associated; often 

 measuring more than one and three-quarters inches in transverse diameter, while 

 the largest specimens of O. vanuxemi seldom measure more than one inch. It differs 

 also in the character and strength of the radiating strige; the muscular imprint of 

 the ventral valve is usually broader and more strongly marked; the cardinal and 

 brachial processes of the dorsal valve are stronger, and directed towards the op- 

 posite valve, while those of 0. vanuxemi are inclined forward or into the cavity of 

 the dorsal valve. 



Geological formation and locality. Shales of the Hamilton group : Western 

 New-York. 



1 2 



Fia. 14 2. Dorsal and ventral valves of Orthis penelope. One of these figures was used in the 

 Regents' Report of 1847, to illustrate Orthis vanvixemi, with which this species was 

 at that time included- 



