128 



HISTORICAL PAL/EONTOLOGY. 



such as the huge and thick-shelled Megalomi of the American 

 Wenlock formation, the Bivalves (Lamellibranchiata) present 



Fig, "jo.Pentamenis Knightii. 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 Cardiola (fig. 71, A and C) and Pterinea (fig. 



Fig. 71. Upper Silurian Bivalves. A, Cardioln intemtpta, 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 Euomphalus (fig. 72, b\ 

 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/), with its thin fragile shell often hardly coiled up 

 at all its minute spire, and its widely-expanded, often sinuated 

 mouth. The British Acroculia should probably be placed 

 here, and the group has with reason been regarded as allied 

 to the Violet-snails (lanthina] of the open Atlantic. The 



