208 BRITISH PAL/EOZOIC FOSSILS. [Bracnropopa. 
shire ; Lower Ludlow mudstone of Green Quarry, Leintwardine, Shropshire; Wenlock limestone of Wenlock ; 
in the Upper Ludlow rock of Presteign, Radnorshire; Upper Ludlow limestone of Burton and Brockton, 
Wenlock. 
Genus. PENTAMERUS (Sow.) 
Gen, Char.—Shell globose, ovate, receiving valve largest ; generally destitute of mesial fold, but the sinus 
when it exists is usually in the entering valve, and the ridge in the receiving ; no hinge-line, area large, unde- 
fined, and having a deep triangular pit in the centre, under the beak of the dorsal valve, and into which the 
beak of the entering valve is strongly incurved ; internally the receiving valve has one large bipartite central 
septum, the walls of which suddenly divaricate as they approach the entering valve, forming the walls of 
the external triangular opening, and enclosing between them a triangular chamber much smaller than the 
two lateral ones; in the entering valve the two corresponding plates are subparallel, and separate from 
their origin, being so curved that internal casts shew one of their edges, like the diverging cardinal teeth 
of Orthis, and the inner edges form the long subparallel slits (casts shew a narrow cardinal area in this 
valve), the middle one of the three resulting chambers being much the narrowest. 
The forms of the lines of growth on the internal plates, their relative distance, and the proportion of the 
central to the lateral chambers in depth of the receiving valve, afford specific characters of great value. 
PENTAMERUS GALEATUS (Dal. Sp.) 
Ref. and Syn.=Atrypa galeata (Dal.) Vet. Acad. Handl. t. 5. f. 4. Sil. Syst. t. 12. f. 4. 
Sp. Ch.—Longitudinally ovate, very gibbous, receiving valve much arched, beak blunt, incurved, nearly 
in contact with that of the opposite valve, apical angle about 75°; in old specimens the margin is raised 
into a wide square ridge, reaching only a short way from the margin; lateral margin nearly level, except 
a small rounded wave toward the receiving valves at the cardinal angles; entering valve depressed, convex 
near the beaks, sides flattened, anterior half depressed at the margin into a sinus; surface with minute 
close concentric undulations, and very variable longitudinal ridges, seldom reaching the beak. Length one 
inch, width the same, proportional length of entering valve =, proportional depth of both valves 75; the 
mesial line of the septa in the receiving valve scarcely one-third the length of the valve, diverging parts 
very small scarcely one-third the depth of the septum; two septa in the entering valve not half its length, 
thin, diverging at an angle of about 7°. 
There are sometimes two or four subequal obtuse plaits in the sinus, extending half way to the 
beak, three or four undulations of each lateral margin, and the rest of the shell free of them, or, in 
other cases, they amount to twelve or fifteen distinct lateral and medial ridges reaching more than half 
way to the beak, and either subequal and simple, or very unequal and branched, in all cases being 
however most distinct on the middle of the shell. 
Position and Locality—Common in the Wenlock limestone and shale of Dudley, Staffordshire, and 
in the Lower Ludlow rock of Garden Quarry, Aymestry, Herefordshire. A smaller variety, about one- 
seventh wider than long (var. globosus), more rounded, and with very unequal plaits, occurs in the 
Aymestry limestone of Leintwardine, Shropshire; Ludlow rock above Parklane, Llandeilo; in greenish 
mudstone, and in Wenlock limestone of Wenlock, Shropshire ; not uncommon in the Wenlock lime- 
stone of Woolhope. 
PENTAMERUS ?GLOBOSUS (Soi. Sp.) 
Ref. and Syn.=Atrypa id. Sow. Sil. Syst. t. 22. f. 25. 
Sp. Ch.—Rounded, subrhomboidal, globose, or when old subeuboid from the deflexion of the front; 
a very shallow undefined mesial depression in the large valve, forming a shallow wave in the margin towards 
the opposite valve, which is evenly convex without a corresponding mesial ridge; lateral margins nearly 
even; apical angle 115°: casts of the receiving valve coarsely punctured about the rostral portion; beak 
