Bracutopopa. | DEVONIAN MOLLUSCA. 383 
HEMITHYRIS SUBDENTATA (Sow. Sp.) 
Ref. and Syn. = Atrypa subdentata Sow. Geol. Trans, 2nd Series, Vol. V. t. 54. f. 7. 
Sp. Ch.—Longitudinally ovate or subtrigonal (the two sides are broken off in Sowerby’s figured specimen), 
with age the front becoming rather produced and elevated into a semielliptical sinus; entering valve con- 
siderably deeper than the receiving one; sides and rostral portion moderately convex and smooth ; front towards 
the margin raised into an undefined broad rounded mesial ridge, bearing three obtusely-rounded ribs, becoming 
obsolete before reaching the beak, and forming angular indentations in the edge; receiving valve gently convex 
on the sides and near the beak, which is small and prominent, with the opening beneath it; middle portion 
towards the margin depressed into a wide deep sinus, containing two thick subangular ridges, which become 
very approximate, and extend obscurely nearly to the beak: surface smooth, densely fibrous under the lens. 
Length six lines, greatest width near the front about the same, depth of both valves 5;. 
Count Munster figures this species (in his Beitriige for 1840) from the black limestone of Schubelhammer 
as the Wenlock Atrypa rotunda of the Silurian system, from which it is certainly distinct by its more elongate 
form, angular teeth in the margin from the ends of the ridges, ec. 
Position and Locality —Rare in the dark limestone of Petherwin. 
Genus. STRINGOCEPHALUS* (De/rance). 
Gen. Char.—Ovate, shell thick, fibrous, smooth ; receiving valve with a large pointed beak (often a little 
oblique to the right side); cardinal area defined, flat, triangular, with a triangular deltidium, which encircles a 
round opening (which is finally filled up in the adult, according to Bronn, &c.) ; the interior of the receiving 
valve has a narrow mesial septum, and two dental lamelle diverging rapidly to a little in front of the cardinal 
angles, between which and the plate which is an internal prolongation of the cardinal area, a small chamber 
results on each side; entering valve with a large mesial septum curving towards the opposite valve, and com- 
posed of two diverging parts at its free edge (like that of the large valve of Pentamerus). 
The pointed instead of inrolled beak, and cicatrised deltidium enclosing a foramen, distinguish this genus 
from Pentamerus, as well as the mesial septum in the entering valve, and the opposite valve having a simple, 
narrow, mesial septum, and very divergent instead of converging dental lamellz. 
STRINGOCEPHALUS GIGANTEUS (So7-) 
Ref—Sow. Geol. Trans. 2nd Series, Vol. V. t. 56. f. 10, 11. 
Sp. Ch.—Transversely oval, depressed, a small sinus in front margin; beak of entering valve small; car- 
dinal area flat, low, triangular. Width three inches nine lines, length three inches, depth of both valyes one 
inch three lines. 
I regret that having only one bad specimen at my disposal, I am unable to say more than that I have 
ascertained that the two diverging septa figured by Sowerby really exist, penetrating about two lines deep into 
the cavity of the shell ; consequently the species cannot be a Stringocephalus. I have seen casts of the same shell 
in the Devonian shale of Looe, shewing the beak between these septa, two cardinal teeth, and a long bilobed 
pair of muscular impressions, as in Orthis. 
If, as I suspect, there are two diverging lamellz in cach valve, the fossil would approach Porambonites ; 
but I have seen no punctured structure, and must, unwillingly, leave the matter undetermined for want of 
materials. I may add, that the great flatness of the valves would alone shew (as Mr Sowerby suggested) that, 
even externally, there is no specific relation between this shell and the Stringocephalus Burtini of the Eifel, with 
which most authors unite it. 
Position and Locality —Devonian limestone of Plymouth. 
* Strigocephalus, according to the spelling of Defrance and many others. 
