234 



MANUAL OF THE MOLLUSCA. 



by a pedicle when young, the impressions of the pedicle-muscle blending with 

 those of the hinge-muscles (c) in the ventral valve. A few species appear to 

 have been permanently tixed. P. striata is irregular in its growth, elongated 

 and tapering towards the beak, and occurs in numbers packed closely to- 

 gether. P. proboscidea seems to have lived habitually in cavities, or half- 

 buried in mud, as suggested by M. D'Orbigny ; its ventral valve is prolonged 

 several inches beyond the other, and has its edges rolled together and united, 

 forming a large permanently open tube for the brachial currents. The large 

 spines are most usually situated on the ears of the ventral valve, and may 

 have served to moor the shell ; being tubular they were permanently suscep- 

 tible of growth and repair. Although edentulous, the dorsal valve must have 

 turned on its long hinge- line with as much precision as in those genera 

 which are regularly articulated by teeth. 



Fossil, 60 sp. Devonian — Permian. N. and S. America, Europe^ 

 Spitzbergen, Thibet, Australia. 



Fig. 154. Exterior. Interior. 



Sah-genus, Aulosteges, Helmersen. A. Wangenheimii, Vern. fig. 154. 

 Permian, Russia. Shell like Producta ; ventral valve with a large flat trian- 

 gular hinge-area (/^), with a narrow convex pseudo-deltidium {d) in the 

 centre : beak a little distorted, as if attached when young ; dorsal valve 

 slightly convex near the umbo ; interior as in Producta {longi-sptna.) 

 Strophalosta, King. 



Ex. S. Morrisii, King. fig. 155. 



Si/n. Orthothrix, Geinitz. 



S/iell attached by the umbo of the ventral 

 valve ; sub-quadrate ; covered with long slen- 

 der spines; valves articulated, dorsal mode- 

 rately concave, ventral convex, each with a 

 smaU area ; fissure covered ; vascular impres- 

 sions conjoined, reniform. 



Fossil, 8 sp. Devonian — Trias. Europe; 



Himalaya (Gerard), 



Fig. 155. S. ^forrisii. 



