Bracutopopa.] UPPER PALAOZOIC MOLLUSCA. 439 
4th Var. mEsocona (Phill.) See page 381. 
Position and Locality —Rare in the lower carboniferous limestone of Derbyshire. 
HEMITHYRIS ANGULATA (Linn. Sp.) 
Ref. and Syn. = Anomia angulata Linn. Syst. Nat.= 7. excavata Phill. Geol. York. Vol. II. t. 12. f. 24. 
Dese—Longitudinally subtriangular, greatest width along the front margin; posterior lateral margins of 
both valves depressed, forming a broad, oval, concave space, extending from the beak rather more than half the 
length of each side; lateral and front margins nearly in one plane in young specimens five lines long, after 
which the whole front becomes greatly raised into a deep subquadrate sinus. Entering valve rather flattened 
except at the beak, which is very obtuse and abruptly incurved; profile nearly straight, very slightly arched, 
the long posterior lateral margins abruptly deflected nearly at right angles; surface even for about one to two 
* lines from the beak, after which three very large acutely-angular ridges become developed, extending to the 
wide sinus in the front margin, which they very deeply indent with acutely-angular teeth, the middle ridge 
slightly larger than the lateral ones; intervening hollows very sharply angular, slightly larger than the ridges ; 
two slightly smaller, similar, large, acutely-angular ridges on each side. Receiving valve flattened in the young 
shell of five lines long, with a nearly straight profile, beyond which all the middle portion is semicircularly 
arched to fill the large sinus in the front margin, the narrowed portion corresponding to the two lateral plaits 
on each side remaining straight; beak very large, prominent, pointed, scarcely incurved ; posterior lateral 
margins abruptly deflected at right angles to form part of the ovate depression on each side, which is defined 
by a sharp angulation; foramen triangular, moderate ; lateral margin having a very small curve towards the 
entering valve near the beak, but a strong semicircular curve towards the receiving valve in the remainder of 
the depressed lateral portion ; edges between the plaits very deeply and angularly indented. Shell-tissue coarsely 
fibrous under the lens, marked, principally in the hollows between the ridges, with longitudinal obtuse striz, 
about ten in one line, crossed by faint, obtuse, concentric lines of growth, about equalling them in size. Length 
of average specimen ten lines, proportional length of entering valve ;j,, width =, width of sinus in front 
52 50 47 
margin +, depth thereof =, length of posterior, lateral, depressed spaces 5, depth of entering valve (greatest 
60 2 
near the front) =, depth of receiving valve %; length of young specimen six lines, proportional length of 
90 
entering valve ;%, length of posterior lateral depressed spaces ,j;,, width ;;;, depth of entering valve and depth of 
sinus ;,, depth of receiving valve ;. 
This very remarkable species is not likely to be confounded with any other. The original specimen 
described by Linnzeus, and ticketed in his handwriting, is in the collection of the Linnzean Society of London, 
Young specimens are depressed, nearly equivalve, the outline being an isosceles triangle, without sinus in the 
front margin, and with the great angular ridges only marked at the edge: with age the sinus in the front margin 
and depth of the entering valve increase very rapidly. 
Position and Locality.—Rare in the carboniferous limestone of Yorkshire; very common in the dark 
lower carboniferous limestone of Isle of Man. 
HEMITHYRIS FLEXISTRIA (Phill. Sp.) 
Ref. and Syn. = Terebratula flexistria Phill. Geol. York. Vol. II. t. 12. f. 33, 34. 
Dese.—Transversely broad-ovate, or rotundato-rhombic; lateral margins nearly straight; sinus in front 
margin abruptly raised, very wide, oblong. Entering valve moderately gibbous, flattened, or slightly concave in 
all the middle portion; beak obtuse, much incurved, tumid, from whence the profile is nearly straight in the 
middle portion, and abruptly arched downwards towards the margin, so that the greatest depth is at about two- 
thirds the length from the beak; sides very abruptly arched downwards, nearly vertical; posterior lateral 
margins with an obscurely-defined, slightly concave, elliptical space (suleated like the rest), extending nearly 
one-half the length; about eight or nine strong, angular ridges raised with the mesial sinus, which only 
interrupts the evenness of the valve close to the margin; about seven to ten slightly larger, very much curved 
[rasc. 111. ] 
