424 Prof. F. M^Coy on some new Brachidpoda 



portional length of receiving valve -f^-^) length of entering 

 valve /q^^, depth of both valves -J-^q^ width of mesial fold y^''^, 

 height of sinus in front margin -i^q, width of cardinal area j-^^ . 



This species is most nearly allied to the >S^. trigonalis, from 

 which it is distinguished by the more spheroidal form, the obtuse 

 rounding of the sides, and the very small number of its lateral 

 ribs j the distinctly ribbed mesial ridge separates it from S, pin- 

 guis, as well as its more depressed form, and the fewer and more 

 prominent radiations. 



Not very uncommon in the carboniferous limestone of Derby- 

 shire. 



(CoL University of Cambridge.) 



Hemithyris heteroptycha (M'^Coy). 



Desc. Transversely oval, very gibbous; hinge-margin obtusely 

 arched, front much elevated into a deep, subquadrate oblong 

 Binus, inclining backwards from the line of the lateral margins 

 at about 80°, which only affects the even convexity of the en- 

 tering valve near the margin ; entering valve evenly gibbous, 

 slightly flattened, or even concave in the mesial line near the 

 beak ; profile abruptly incurved at the beak, and rather ab- 

 ruptly curved downwards near the front margin ; intervening 

 portion nearly straight, the greatest depth being at about two- 

 thirds the length from the beak ; sides very abruptly arched 

 downwards to the lateral margins, which are veiy slightly 

 sigmoid at the sides, the commissure obtusely angular up to 

 the beak, having on each side of the rostral portion a very 

 slight narrow depression ; three mesial ridges occupy the width 

 of the top of the sinus, very large, obtusely angular, deeply 

 indenting the margin, and continued simply to the beak; 

 lateral ribs dichotomizing at from one to two lines from the 

 beak, unequal in size, fifteen or sixteen on each side near the 

 margin, scarcely half the size of the mesial ridges, very 

 strongly arched at the sides ; receiving valve with the lateral 

 third on each side, and the rostral portion slightly convex, 

 the middle portion after about two lines long very abruptly 



• curved downwards into a very wide mesial hollow to fill the 

 oblong sinus in the front margin, the profile of the middle 

 being nearly semicircular, the lateral ridges all bifurcate at 

 about three lines from the beak, the two large mesial ridges 

 simple, having besides a small dichotomous pair on each side in 

 the sinus. Width 6 lines, proportional length of receiving valve 

 about y%%, length of entering valve -f^j^y width of sinus in 

 front margin about -f-^Q, depth thereof y%''^, depth of receiving 

 valve at middle of side y%^^, greatest depth of entering valve 



