PENTAMERUS. 123 
except that our examples have two ribs on the fold instead of three. As, however, 
Davidson figured a small specimen having a triple fold, this variation appears of 
no consequence. When the shell is very aged the ribs become still steeper and 
subangular, and then I can see no difference between it and P. acutolobatus, 
Sandberger. Considering the amount of variation in the kindred P. brevirostris, 
I have little hesitation in grouping these forms together, as they can be traced 
upwards by connecting links. It is, however, to be noted that the ribs are not 
always equally developed as the shell increases in size, some of the larger speci- 
mens retaining to a greater or less degree the comparative smoothness of the 
young form. 
The shell seems covered near the margins with fine sharp growth-lines, which 
follow the zigzag curve of the margins. In two large specimens the apical part 
of the dorsal shell is removed, showing a median plate of a spear-shaped form con- 
sisting of two flat wings and a more prolonged triangular central ridge. 
I cannot myself see any distinction between T. Wurmii, F. A. Romer, and 
young forms of this species; but, as Davidson did not identify them, I do not 
venture to do so without further evidence, seeing that the name of the species 
would then have to be changed to P. Wurmit. 
3. PENTAMERUS SUBLINGUIFER, Mawrer? Pl. XIV, figs. 6—8. 
1885. PrNTAMERUS SUBLINGUIFER, Maurer. Abhandl. Grossh. Hessisch. Geol. 
Landes., vol. i, pt. 2, p. 218, 
pl. ix, figs. 7—10. 
Description.—Shell small, subcircular, flattish when young, subglobose when 
old. Ventral valve slightly convex, prolonged in the centre into a very shallow 
tongue-like sinus; beak small, low, incurved. Dorsal valve more or less convex, 
with an indistinct central fold, bounded by slight furrows. Apex of dorsal valve 
more or less approaching the beak in size and prominence. 
Lovality.—There are thirteen specimens in my Oollection, and six in the 
Woodwardian Museum from Lummaton. 
Size.—Height, 19 mm.; width, 19 mm.; depth, 15 mm. 
Remarks.—My fossils are in two sets, both of which seem to fall within the 
limits of P. sublinguifer. 
(1) There are eleven very small shells with almost equal beaks and wide flat 
folds, which are generally turned up in front, and are bounded by indistinct furrows. 
Though the apices are nearly equal, that of the sinus-bearing valve seems slightly 
