MANUAL OF THE MOLLUSGA. 
Type, Pentamerus lens, Sowerby. 
Sheil usually large, elongate-oval, &.; valves nearly equal, 
never globose ; a short mesial septum in the interior of the yen- 
tral valve supporting a small triangular chamber beneath the 
beak asin 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. levis 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 sheli 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. ; 
Faminy IV.—ORTHIDz.* 
SKENIDIUM, Hall, 1861. 
Hiymology, skenidion, a little tent. 
Type, Orthis insignis. 
Shell haying 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. 
Streptorhynchus pelargonatus. 
Fig. 25.—Interior of the ventral valve; ¢, teeth ; a, cardinal museular impressions. 
Fig. 26.—lnterior of the dorsal valve ; s, sockets ; v, cardinal process ; 7, adductor scar, 
Etymology, strepto, I bend or twist; rhynchos, a beak. 
Tynes, 8. pelargonatus, Schloth. sp. ; 8. Devonica, D’Orb ap. 
* See p. 379. 
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