303 

 ORTHIS ? ARMANDA. (N. sp.) 



b. c. 



Fig. 293. Fig. 294. 



Fig. 293. Orthis Jlrmanda. a, ventral valve, of a wide variety ; 6, ventral valve of a 

 narrower form, showing a cast of the interior of the beak and umbo ; c, 

 dorsal valve. 

 $94. Orthis Minna. 



Description. Shell of medium size, varying in width ; hinge line 

 straight; cardinal angles 90, or often a little less; sides straight, v and 

 either parallel or a little converging from the hinge line forward for one- 

 fourth or one-half the length ; anterior angles and front margin rounded. 

 Ventral valve pyramidal, varying in height, the beak usually the most 

 elevated point, thence with a convex or straight slope to the anterior 

 angles and margin, the slope to the cardinal angles either flat or gently 

 concave. Dorsal valve depressed convex, concave towards the cardinal 

 angles ; beak scarcely elevated above the hinge line. 



The area of the ventral valve is rather large, sometimes three lines in 

 height, often less, very variable in this respect, either at right angles to 

 the plane of the margin, or inclined thereto sometimes as low as 45. 

 Foramen moderate, apparently open. In the interior the dental plates 

 seem to form an imperfect triangular chamber, thus differing somewhat 

 from the usual forms of the genus. 



Surface with fine sharp distinct striae several times sub-divided, from 

 six to ten in the width of one line at the margin. These striae are usually 

 crenulated by concentric lines, especially towards the front margin. 



Width on the hinge line from 6 to 10 lines ; length varying from 

 one-half to three-fourths of the length. 



Locality and Formation. Phillipsburg (limestone), B. 2, section 

 Geol. of Can., p. 844 ; Quebec group. 



Collectors. E. Billings, Dr. P. J. Farns worth. 



ORTHIS MINNA. 



Fig. 294. 



Description. Shell rather large, sub-quadrate ; hinge line straight, 

 equal to the greatest wiSth ; cardinal angles rectangular ; sides in the 

 upper half straight and parallel ; front angles and margin broadly rounded. 



