128 HISTORICAL PALEONTOLOGY. 



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

 Wenlock formation, the Bivalves (Lamcllibranchiatd) present 



Fig. jo.Pentamertis 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, Cardiola intemifta, 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 fe\v charac- 

 teristic forms are here figured (fig. 72). Of these, no genus 

 is perhaps more characteristic than Euomphahis (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 andy), with its thin fragile shell often hardly coiled up 

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

 mouth. The British Acroculicz 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 



