440 BRITISH PALAZOZOIC FOSSILS. [Bracutopopa, 
ridges on each side; those of the middle simple, those of the sides occasionally duplicate near the margin, all 
continued simply to the beak. Receiving valve moderately and evenly convex in the rostral portion for three 
or four lines long, after which the narrow sides are continued straightly to the margin, but the wide mesial 
portion is regularly arched downwards, to fill the mesial sinus; lateral ridges straight; beak very small. 
Average width eight lines, proportional length of receiving valve ,“;, length of entering valve ;;,, width of mesial 
sinus ,”, depth thereof ,{;, depth of entering valve =, depth of receiving valve in middle of side =. 
This species differs from H. plewrodon, in its much more numerous and smaller ridges, and the great 
curvature of the lateral ones of the entering valve, as well as the remarkable depression of all the middle 
portion ; from Terebratula (Camerophoria) sulcirostris and C. ventilabrum it is distinguished by all the ridges 
extending to the beak. The two distinctly separated diverging dental lamelli in the receiving valve, and mesial 
septum in entering valve, are small. 
Position and Locality —Not uncommon in the carboniferous limestone of Derbyshire. 
HEMITHYRIS HETEROPTYCHA (JM°Coy). PI. 3. D. fig. 19. 
Ref.—Id. id. M°Coy, Annals of Nat. Hist. 2nd Series, Vol. X. 
Desc.—Transversely oval, very gibbous ; hinge-margin obtusely arched ; front much elevated into a deep, 
subquadrate, oblong sinus, inclining backwards from the line of the lateral margins at about 80°, which only 
affects the even convexity of the entering valve near the margin. Entering valve evenly gibbous, slightly flat- 
tened, or even concaye, in the mesial line near the beak; profile abruptly incurved at the beak, and rather 
abruptly 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 very 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 dichotomising 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. 
85 
Width six lines, proportional length of receiving valve about 4, length of entering valve “,, width of sinus in 
1009 
front margin about ;;, depth thereof ;;;, depth of receiving valve at middle of side %, greatest depth of entering 
valve ;%; the three mid-ridges at margin occupy a space of two lines, six of the lateral ones occupy the same 
space; surface smooth. 
The great number of small, obtuse, dichotomising plaits on the sides distinguishes this from any of the 
varieties of H. flexistria, C. ventilabrum, &e., as well as their extending quite to the beak, and the dispropor- 
tionally large mesial ridges, separate it from the H. bifera and H. cuboides. I have not seen the dental 
lamellee. 
Position and Locality—Rare in the carboniferous limestone of Derbyshire. 
Explanation of Figures.—P\. 3 D. fig. 19, front view, natural size; (the two mesial ridges should be 
slightly wider, and the lateral ones slightly narrower) ; fig. 19 @, ditto receiving valve; fig. 19 4, profile of ditto ; 
(the top of the sinus should incline slightly more towards the beak, and the middle behind the front edge 
should be more elevated from the front, and flattened in the middle) ; fig. 19 c, view of entering valve of ditto 
magnified two diameters, to shew the difference in character between the mesial and lateral ridges. 
HEMITHYRIS LONGA (M°Coy). PI. 3. D. fig. 24. 
Ref.—ld. id. M°Coy, Annals of Nat. Hist. 2nd Series, Vol. X. 
Desc.—Longitudinally trigonal, length exceeding the width or depth, greatest width at one-third from the 
front margin; lateral anterior margins obtusely rounded ; front very obtusely angulated in the middle; posterior 
