444 Fossils of the Chazy Limestone. 



Description. — Triangular, varying from moderately convex to 

 sub-globular, apical angle from 80° to 100°; sides straight or 

 nearly so for about two-thirds the length ; greatest width at one- 

 fourth or one third from front ; front angles truncated ; front 

 straight or rounded. The ventral valve in the upper half is nar- 

 row or somewhat acute, with the beak in the more slender indi- 

 viduals prominent, and moderately arched, but in the globose 

 forms incurved down to the umbo of the dorsal valve ; in the 

 front half strongly elevated at the angles on each side of the sinus ; 

 the latter is deep in front, but dies out at a little more than half 

 the length. Dorsal valve the most convex ; beak closely incurved ; 

 the umbo rather broad, rounded, divided into lobes by a narrow 

 sulcus which extends from the beak one-fourth or little more of 

 the length downwards, a strongly elevated mesial fold extending 

 a little more than half the length. Surface with about nine acutely 

 angular, strongly elevated ribs, of which there are usually three in 

 the sinus and four in the mesial fold. The upper half of the sheU 

 is compressed laterally, so that just below the beak on each side 

 there is a flat or concave oval space, which is smooth in the centre, 

 but sometimes exhibits two or three small ribs on each valve. I 

 have not detected any concentric strise. Length three or four 

 lines ; width, either equal to or a little greater than the length. 



This little shell is quite distinct from R. plena, differing there- 

 from in its smaller size and proportionally larger ribs. R. plena 

 has from 15 to 20 ribs ; bnt this species has only 9 or 10, exclud- 

 ing the small and short ones on the sides. It is found in great 

 abundance in certain beds of the Chazy limestone at the Mingan 

 Islands. I have seen no specimens of R. plena from that locality, 

 and this species appears to be its representative there. 



Locality and Formation. — Mingan Islands. Chazy Limestone. 



Collectors. — Sir W. E. Locran and J. Richardson. 



Rhynconella plena. (Hall, S.p.) 



Atrypa plena, A. altilis, A. plicifera (Hall), Palaeont., N. Y. Vol. V 

 p. 21, 22, 23. Plate 4 bis. 



h 



Fig. 22. 

 Fig. 22.— A large specimen of R. plena. Those that are found in such 

 great numbers at Montreal are in general smaller than the above. 



