MANUAL OF THE MOLLUSCA. 



Type, Pentamerus lens, Sowerby. 



Shell usually large, elongate -oval, &c. ; valves nearly equal, 

 never globose ; a short mesial septum in the interior of the ven- 

 tral valve supporting a small triangular chamber beneath the 

 beak as in Pentamerus ; in the dorsal valve no longitudinal septa, 

 spires, or loop, the whole of the internal solid organs consisting 

 of two short or rudimentary dental plates, which in some spe- 

 cies bear prolonged calcified processes for the support of the 

 cirrated arms. A more or less developed area in the ventral valve. 



In S. Icevis and S. microcamerus the hinge line is straight and 

 much extended. In S. Arachne, Billings, the area of the ven- 

 tral valve is so much developed as to give the whole shell the 

 external appearance of an Orthis. 



Distribution,' 10 species. Middle Silurian, Europe, America. 

 S, elongata, Vanuxem, is the only species known in the Devonian 

 rocks. 



Family IY. — Ohthid^.* 



SzEmDiuM, Hall, 1861. 



Etymology, sJcenidion, a little tent. 



Type, Orthis insignis. 



Shell having the general aspect of Orthis, except in the ex- 

 treme elevation of the ventral valve ; cardinal process prolonged 

 into a median septum, which extends to the base or front mar- 

 gin of the shell, and occasionally bifurcates at this lower ex ■ 

 tremity. Area large and triangular in the typical species. 



Distribution, 3 species. Silurian, United States. 



Streptorhynchus, King, 1850 (see p. 380). 



Fig. 25. Fig. 26. 



Streptorhynckus pelargori'itus. 

 Fig. 25. — Interior ot the ventral valve ; t, teeth ; a, cardinal muscular impressions. 

 Fig. 26. — luterior of the dorsal valve ; s, sockets ; v, cardinal process ; r, adductor scar. 



Etymology, strepto, I bend or twist ; rhynchos, a beak. 

 Types, S. pelargonatus, Schloth. sp. ; S. Devonica, D'Orb. sp. 



* See p. 879. 



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