CAMBRO-SILURIAN FOSSILS. 215 



Very rare in the Upper Ludlow rock of Burton and Brockton 

 near Wenlock. 



(Col. University of Cambridge.) 



Hemithyris nasuta (M^Coy). 



Sp. Char. Longitudinally ovate, longer than wide, gibbous, en- 

 tering valve much deeper than the receiving one ; receiving 

 valve with a small obtuse beak, incurved nearly to touch the 

 beak of the entering valve, with a small triangular perforation 

 beneath ; rostral portion tumid for about 5 lines from the 

 beak, beyond which a wide flattened mesial depression is de- 

 veloped, gradually deepening towards the front, which in old 

 specimens is very much produced into a flat tongue-shaped 

 lobe, nearly as long as wide, gently sloping to the level of the 

 most convex part of the entering valve ; sides obtusely defined 

 from the mesial furrow, gently convex, lateral margins slightly 

 sigmoid, and bent upwards at the front, at a rounded angle of 

 about 100° : entering valve with a slightly prominent beak ; 

 rostral portion and sides gently convex, after about 5 lines 

 from the beak the middle is prolonged nearly in a straight 

 line to the produced front margin, forming a large obtusely 

 rounded mesial ridge : surface radiated with simple, close, ob- 

 tusely rounded ribs, about four of which, rather larger than 

 the rest, are raised with the mesial furrow, the broad sides of 

 which are smooth, each side with about twelve slightly smaller 

 ribs (seven in two lines at six lines from the beak near the side 

 of the mesial furrow), leaving a rather broad smooth space at 

 the rostral lateral margin on each side (a fine longitudinal stri- 

 ation is seen in some places). Length 1 inch 2 lines, propor- 

 tional length of entering valve -^^q, greatest width, about the 

 middle of the length, -^-^-^^ greatest depth of both valves (one- 

 third from the beak) -^-^q. 



In form this species much resembles the Terebratula promon- 

 torium of Kutorga from the Lower Silurian limestones of Pulkowa 

 (see Verhandlungen der Buss. Kais. Min. Gesellschaft zu St. 

 Petersburg for 1845, t. 6. f. 3), but is distinguished by its ribbed 

 surface. 



Not uncommon in the schistose limestone of Craig Head near 

 Girvan, Ayrshire. 



{Col. University of Cambridge.) 



Hemithyris sphceroidalis (M'Coy). 



Sp. Char. Suborbicular, gibbous, nearly equivalve, margins very 



obtuse from the inflection of the edges ; lateral margins 



nearly level, front margin abruptly raised into a quadrate 



sinus, about twice as wide as long ; receiving valve with a very 



