Bracuropopa.] LOWER PALAZOZOIC MOLLUSCA. 209 
with a short strong slit (of the medial plate), rapidly divaricating inwards to enclose the beak of the 
entering valve; sides marked with four or five obscure longitudinal ridges (probably of the ovaries). 
Average width of small specimen seven lines, proportional length of receiving valve =; depth of receiving 
valve *, increasing greatly with age. 
Position and Locality.—In the schists above the second grits, Keeper’s Lodge, Goldengrove, Llandeilo ; 
fine Bala sandstone of Beavers Grove, Bettws-y-coed, N. Wales. 
PrentTaAMERus Knicuti (Sow.) 
Ref. and Syn. = P. Knighti+ P. Aylesfordi Sow. Min. Con. t. 28, and Sil. Syst. t. 6. f. 8. 
Sp. Ch.—Small valve rotundato-trigonal, evenly and moderately convex; large valve only little more 
convex than the opposite one when one inch long, but with age becoming rapidly more prominent, into a 
large conical incurved beak; average apical angle about 85°; cardinal area smooth, very variable in size, with 
an obtuse edge; septa of the small valve diverging at an angle of 12°, about half the length of the 
mesial septum in the receiving valve; septum of the large valve extending from the beak nearly to the 
margin, medial flat portion with a concave terminal edge (and having concave lines of growth), about a 
fifth wider than the divaricating portions which form the hood-shaped chamber connected with the triangular 
opening under the beak; front and lateral margins level; surface radiated with about forty subequal, obtusely 
angular ridges in a specimen two inches long (increasing to about sixty, and becoming irregularly alternate 
in size at the margin of a specimen four inches long). Length of rather small specimen two inches four 
lines, proportional length of entering valve =, width =. 
The beak of the large valve is often slightly oblique, and five or six of the ridges in the middle of 
the valves are often less than the others. 
Position and Locality——Very abundant in the Aymestry limestone near Aymestry, Herefordshire, and 
in same rock at Leintwardine, Shropshire; ditto, Sedgley; in the green mudstone (Lower Ludlow rock) 
of Garden Quarry, Aymestry, Herefordshire; very common in the Aymestry limestone near Woolhope, 
with an extremely elongate, narrow variety, with slightly thicker and fewer ribs (var. elongatus,) length 
three and half inches, proportional width * 
100° 
PENTAMERUS LEVIS (Sov.) 
Ref—Sow. Min. Con. t. 28. Sil. Syst. t. 19. f. 9. 
Sp. Ch.—Transversely oval, both valves evenly convex, smooth, or with few faint, broad, radiations near 
the edge; front and lateral margins level; apical angle about 115°: casts shew in the receiving valve the wide 
slit left by a very thick medial septum extending nearly to the front edge (the divaricating rostral portion 
much larger than in the P. oblongus); entering valve with two long thin septa diverging at about 10°, 
reaching more than half way to the front margin, the nearly horizontal curved outer edge, forming the 
large slits parallel with the hinge-margin, having a faint mesial septum between them reaching within 
one-half or one-third of the beak. Width one inch eleven lines, proportional length ;, length of entering 
valve #, depth of receiving valve 3. 
Position and Locality.—Abundant in the sandstone of Pwllheli, Caernarvonshire; Upper Bala sandstone 
of Penlan, Llandovery; Upper Bala flag of Cyrn y Brain, Wrexham, Denbighshire ; ditto 3 miles N. of Builth, 
Radnorshire; Caradoe sandstone and limestone of Soudley, Shropshire. Not uncommon in the Caradoc 
sandstone of Horderly. 
PENTAMERUS LENS (Sow. Sp.) 
Ref. and Syn.= Atrypa lens Sow. Sil. Syst. t. 21. f. 3. 
Sp. Ch.—Longitudinally ovate, moderately convex, greatest width slightly nearer the beak than the 
front; a large, obtusely rounded mesial ridge from the beak of the small valve, widening greatly towards the 
front margin, which is prolonged into an obtuse tongue-shaped lobe, margins nearly level ; surface crossed by 
sharp small lines of growth, with occasional obscure indications of fine longitudinal lines (?internal); casts 
[rasc. 11. | Ee 
