891 



Rhyn. 



ruptly rounding, and at the base produced in a slight exten- 

 sion. Ventral valve depressed convex, slightly gibbous at 

 about one-third the length from the apex, curving abruptly to 

 the cardinal and antero-basal margins, more gradually to the 

 lower lateral margins, becoming depressed in a shallow sinus, 

 which commences at about one third to one-half the length 

 from the beak ; beak slightly incurved, strongly projecting be- 

 yond the beak of the opposite valve. Dorsal valve convex, 

 greatest convexity at or a little below the middle, gradually 

 curving to the apex and cardinal margins, more abruptly to the 

 lateral margins; mesial fold commencing at a little above the 

 middle and often becoming prominent at the front; beak small 

 and nearly straight. Surface marked by from ten to twelve 

 rounded or subangnlar plications, which become obsolete on 

 the upper portions of the valves. The mesial fold is composed 

 of three plications much more prominent than the others. 

 In the sinus there are three plications smaller than the 

 others; the two plications bordering the sinus larger than 

 tho?e on the other portions of the shell. The specimens ob- 

 served have a length of 9 or 10 mm., and a width of from 7 to 

 9 mm.; a transverse section broadly oval, having a width of 5 

 or 6 mm. This species is easily distinguished from any other 

 of this formation at present known. The outline is very sim- 

 ilar to some of the forms of U. {Stenocisma) eximia^ of the 

 Chemung group, but the shell is smaller, the plications of the 

 fold and sinus are fewer and the plications become obsolete on 

 the upper half of the valve. Clinton, Vh. 



Rhynchonella laticostata. For Hgures see under Atrypa 

 laticostata^ where they are wrongly so named. (R. P. W., 1889.) 



Rhynchonella ? laura, Billings. ( A synonym for liei- 



orhynchus multi- 

 costus. S. A. Miller) 

 Geology of Canada, 

 1863, page, 384, fig. 418 

 a^ dorsal; J, side; e, 

 ventral. Hamilton of 

 Canada. VIII c, 



Rhynchonella lepida. Claypole's specimen 12,878, from 

 the King^s mill sandstone^ Perry Co., Pa. (Gr. B. S.) VIII-XI 



