Bracutoropa. ] LOWER PALMOZOIC MOLLUSGA. 211 
PENTAMERUS OBLONGUS (,So7.) 
Ref—Sil. Syst. t. 19. f. 10. 
Sp. Ch.—Longitudinally subtrigonal or ovate, both valves moderately convex, the middle third forming 
a wide, obscurely marked mesial elevation in each valve ; surface smooth near the beak, radiated with obscure 
broad, irregular furrows towards the margin; apical angle about =; front and lateral margins level ; casts 
of the large valve shew a strong narrow medial slit extending three-fourths the length of the shell, the 
divaricating portions and included chamber of the beak very small (little more than one-third the height 
of the medial part), and its width rather less than half the height; in the small valve the septa reach 
little more than one-third the length of the shell, diverging at an angle of 12°. Length two inches three . 
lines, proportional length of entering valve =, width (a little in front of the middle) “, depth of both 
valves =. 
Position and Locality—Abundant in the Caradoc limestone of Acton Scott, Church Stretton, Shrop- 
shire; in the sandstone near Presteign, Radnorshire ; schists of Castell Craig, Gwyddon, Llandovery, S. Wales; 
schists of Goleugoed, Llandovery, Mandinam, Caermarthenshire ; Caradoc limestone, the Hollies, Church Stret- 
ton, Shropshire ; abundant in the Caradoc sandstone of Horderly. 
PENTAMERUS UNDATUS (Sow. Sp.) 
Ref. and Syn. = Atrypa id. Sow. Sil. Syst. t. 21. f. 2. 
Sp. Ch.—Transversely oval, gibbous, large valve most so, with a very wide, flattened, strongly defined 
hollow from the beak, widening towards the margin, which it elevates into a strong obtuse lobe in the small 
valye, on which a corresponding flattened, prominent, strongly marked ridge extends from the margin to 
the beak; sides convex, gradually sloping to the cardinal angles, which when well preserved are nearly 
rectangular, from the straight hinge-line extending almost the width of the shell, but more usually appear 
(from injury) elliptically rounded; beak moderate, apical angle (including the cardinal angles) 140°;  sub- 
stance of the shell very thick; casts very often shew a coarse longitudinal, occasionally branched sulcation, 
indicative of the ovaries; the beak of the large valve with a very short mesial septum (about one-fifth of 
the length of the shell), divaricating to include the beak of the small valve, which has a very minute diverging 
plate on each side of the beak, exterior to each of which is a transverse oval depression; surface smooth, 
or with fine lines of growth. Average width of small specimen eleven lines, proportional length of receiving 
valve =, of entering valve nearly the same, depth of both valves =. 
The internal plates are excessively small, but formed on the usual plan of Pentamerus. I observe that 
in those species in which the plates are very small, the outward-curved marginal portion of the two diverging 
plates in the beak of the entering valve is unusually strong, giving to casts the appearance of four slits in 
the beak, or of two slits and two oblique teeth. 
Position and Locality —Very common in the greyish Bala schists of Mathyrafal, S. of Meifod, Mont- 
gomeryshire ; common in the Bala schists of Pen y Craig, Llangynyw, Montgomeryshire ; Bala schists of 
Goleugoed, Llandovery, Mandinam, Caermarthenshire ; common in the calcareous schists of Penlan, Llando- 
very, S. Wales. 
8th Family. ORTHISIDA. 
Shell fixed, fibrous, or punctured; an opening for the passage of the muscle of attachment between 
the beaks; (arms and other soft parts supposed to resemble Lingula;) a low triangular cardinal area in 
each valve, largest in the receiving valve. No spiral testaceous internal appendages. 
Genera: 1, Porambonites; 2, Orthis; 3, Orthisina; 4, Leptaena; 5, Strophomena; 6, Leptagonia ; 
(7, Chonetes; see Leptena). 
EE2 
