38 



width, indenting the opposite valve, at the margin, to an extent equal to 

 the depth of both valves or nearly so. From the margin it becomes 

 gradually more shallow with an uniform curve until it becomes obsolete 

 at about the mid-length of the shell, or a little above ; a rib on each side 

 slightly longer than the others extend, nearly to the beak, in some speci- 

 mens. There are three ribs only in the depression, in all the specimens 

 collected. The rib on each side presents a large flat, slightly convex or 

 slightly concave face, sloping upwards, and more or less inclined out- 

 wardiB from the bottom of the depression. 



The dorsal valve is rather strongly convex. On a side view (fig., b.) 

 the front margin is nearly squarely truncated ; the outline, from the most 

 elevated point at the front, is sometimes nearly straight for one-third or 

 one-half the length, but in general it is gently convex for that distance 

 and then becomes more or less abruptly curved down to the beak of the 

 ventral valve. The umbo is of moderate size, rounded, abruptly elevated 

 from the beak in large specimens, sometimes with a very faint mesial de- 

 pression for two or three lines. The mesial fold is strongly elevated at 

 the front margin but dies out at the mid-length, or a little above. 



Surface with strong sub-angular ribs ; four on the mesial fold and three 

 in the mesial depression. On each side of the mesial fold and depression 

 there are usually five or six ribs in the smaller and medium-sized speci- 

 mens, and seven, eight or nine in the larger. The most common number 

 is six on each side. 



The ribs are more strongly curved outwards towards their extremities 

 than is represented in the above figures. 



This species is closely allied to H. Formosa, Hall, Pal., N. Y., vol. B, 

 but attains a larger size, and is more coarsely ribbed. 



Locality and Formation. — Grand Greve,Gaspe,Gaspe limestone. No. 8. 



Collector.— B.. Bell. 



Ehynchonella pleiopleura. (Conrad.) 



Fig. 19. Fig. 20. 



Fig. 19.— Dorsal view ; 20, side view. 



