143 



RhYNCHONELLA aLACIALIS. (N. sp.) 



Fig. 120. 

 Pig. 120. — Rhynchcmella glacialis. — a, Side view; 6, dorsal view. 



Description. — Shell rather large, transversely elliptical, greatest width 

 about the middle, length about one-eighth or one-ninth less than the width. 

 Ventral valve varying from moderately to strongly convex, most prominent 

 in the upper half; umbo narrow, sub-carinated ; beak small, closely incurved 

 down to the umbo of the dorsal valve ; mesial sinus wide, concave, extending 

 to the umbo. Dorsal valve about equal to the ventral in the amount of its 

 convexity ; beak hidden beneath that of the ventral valve ; umbo divided 

 by a narrow sinus, which commences under the beak of the ventral valve, 

 and, extending about one Ime therefrom, gradually divides, the branches 

 being obscurely indicated aU the way to the front, on either side of the 

 mesial ridge ; the latter convex and moderately elevated at the front 

 margin, becoming obsolete near the umbo, on which its place is occupied 

 by the sinus above mentioned. Surface of both valves with somewhat 

 obscure bifurcatiag ribs. On the dorsal valve there are two or three ribs 

 in the sinus at the umbo which are extended on to the mesial ridge, and 

 subdivide into from 6 to 8 before reaching the front. The ribs on each 

 side and also on the ventral valve divide in the same way, becoming less 

 conspicuous towards the front. In perfect specimens the surface also 

 exhibits some concentric striae near the front, but in general the ribs have 

 a smooth aspect. 



This species is allied to H. fringilla, and occurs along with it in the 

 same beds, but differs from it in its somewhat smaller size and divided ribs. 



The young of the two species differ in the same respects. 



Locality and Formation. — Gull Cape, Anticosti. In Division 1, An- 

 ticosti group ; Middle Silurian. 



Collector. — J. Richardson. 



