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 fixed. P. striatais 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, 



S/ib-genus, Aulosteges, Helmersen. A. Wangenheimii, Vern. fig. 154. 

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

 gular hinge-area (h} % 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-spina.) 

 STROPHALOSU, King. 



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



Syn. Orthothrix, Geinitz. 



Shell 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 

 small area ; fissure covered ; vascular impres- 

 sions conjoined, reniform. 



Fossil, 8 sp. Devonian Trias. Europe; 



Himalaya (Gerard), 



Fig. 1 55. S. Morrisii. 



