Bracuioropa.] UPPER PALAXOZOIC MOLLUSCA. 44] 
lateral margins long, nearly straight, converging to the beak at an angle of 80°; commissure with a slight wave 
towards the entering valve in the posterior half of the lateral margins; anterior half of the lateral margins 
nearly straight, with a very faint indication of one plait on each side; middle of the front margin elevated at 
an angle of 85° with the plane of the lateral margins, into a wide, tongue-shaped sinus, with sigmoid sides, 
acutely angular in the middle. Entering valve with the profile very slightly arched, greatest depth about the 
middle of the length; sides convex, arched abruptly downwards on each side from the obtuse mesial line. 
Receiving valve flattened, or very slightly convex, for about three lines from the beak, after which the very 
narrow sides alone are continued nearly straight to the obtusely rounded anterior lateral angles, the wide 
mesial portion being strongly depressed, to fill the sinus in the front margin; beak rather large, very slightly 
incurved. Surface smooth, with a few obtuse imbrications of growth near the margin; tissue very coarsely 
fibrous, almost visible to the naked eye. Dental lamelle in beak of receiving valve very short, subparallel, 
slightly divaricating; mesial septum of entering valve very minute. Length nearly six lines, proportional 
width ;%, length of entering valve ;;;, depth of entering valve #4, depth of receiving valve 34, width of sinus “, 
depth thereof >. 
By the great thickening of the margins it is obvious that this little shell is adult. It only approximates, 
in the most remote degree, to one other Hemithyris that I know of, viz. one of the varieties of H. acuminata, 
from which it is distinguished by its very small size, and the length exceeding both the width and the depth. 
Position and Locality.—V ery rare in the carboniferous limestone of Derbyshire. 
Explanation of Figures —PI\. 3 D. fig. 24, front view, natural size; fig. 244, do. profile; fig. 246, do. 
front view of entering valve; fig. 24 c, do. fibrous tissue of the surface, magnified. 
HEMITHYRIS PLEURODON (Phill.) See page 382. 
I might add to what is stated at the above page, that the commissure of the valves is sharply angulated all 
round to the beak on each side. The lateral plaits are sometimes as few as three on each side; there is often 
a shallow, mesial, longitudinal indentation near the beak of the entering valve; and that the length in pro- 
portion to the width varies from jj; to 3. The young at a couple of lines long have the same number of ridges 
as the adult, but are much flattened, with slight trace of sinus, and seem rather longer. The sharp angular 
extension of all the strong simple ridges to the apex of the beak, separates this species from the H. pugnus 
and C. sulcirostris. The two diverging dental lamellze in the receiving valve are moderate in size, but dis- 
tinctly separated throughout; the mesial septum in the entering valve is small. 
Position and Locality—Common in the carboniferous limestone of Derbyshire; not uncommon in the 
carboniferous limestone of the Isle of Man; a variety rare in the carboniferous limestone of Lowick, Northum- 
berland; a variety with three mesial plaits, common in the carboniferous shale of Craige near Kilmarnock ; 
carboniferous limestone of Berwick-on-Tweed. 
HEMITHYRIS RENIFORMIS (Sow. Sp.) 
Ref. and Syn. = Terebratula, id. Sow. Min. Con. t. 496. f. 1 and 2. 
Desc—Transversely reniform or obtusely bilobed. Entering valve very gibbous; profile greatly arched to 
the beak, and rather more abruptly to the sinus in the front margin; mesial ridge square, flattened at top, 
prominent, strongly defined, and bearing three or four large angular ridges, all becoming obsolete at about five 
lines from the beak; sides very tumid, inflated, much prolonged into elliptical lateral lobes, very steeply sloped 
from the sides of the mesial ridge to the lateral margins, which are faintly arched towards the entering valve 
and very abruptly raised into a very deep oblong sinus, inclining backwards, nearly at 125° from the plane of the 
lateral margins. Receiving valve with a very short adpressed tumid beak; a nearly straight convex portion 
extends from the beak to the base of the sinus on each side, from which it slopes rapidly to the lateral margins 
(giving the characteristic appearance to the sides of the receiving valve, of hanging below the edges); very wide 
middle portion depressed, after two or three lines from the beak, into a very wide concave hollow, which receives 
two or three large angular ridges after about nine lines from the beak, which extend to the middle of the sinus, 
3L2 
