Res: HISTORICAL PALASONTOLOGY. 
such as the huge and thick-shelled AZega/omi of the American 
Wenlock formation, the Bivalves (Zamellibranchiata) present 
Fig. 70.—Pentamerus Knightiz. Wenlock and Ludlow. The right-hand 
figure shows the internal partitions of the shell. 
little of special interest ; for though sufficiently numerous, they 
are rarely well preserved, and their true affinities are often un- 
certain. Amongst the most characteristic genera of this period 
may be mentioned Cardvo/a (fig. 71, A and C) and Prerinea (fig. 
Fig. 71.—Upper Silurian Bivalves. A, Cardiola interrupta, Wenlock and Ludlow; 
B, Pterinea subfalcata, Wenlock; C, Cardiola fibrosa, Ludlow. (After Salter and 
M ‘Coy.) 
71, B), though the latter survives to a much later date. The 
Univalves (Gasteropoda) are very numerous, and a few charac- 
teristic forms are here figured (fig. 72). Of these, no genus 
is perhaps more characteristic than Lwomphalus (fig. 72, 6), 
with its flat discoidal shell, coiled up into an oblique spiral, 
and deeply hollowed out on one side; but examples of this 
group are both of older and of more modern date. Another 
very extensive genus, especially in America, is Platyceras (fig. 
72, a and f), with its thin fragile shell—often hardly coiled up 
at all—its minute spire, and its widely-expanded, often sinuated 
mouth. The British Acroculie should probably be placed 
here, and the group has with reason been regarded as allied 
to the Violet-snails (Zanthina) of the open Atlantic. The 
