BRACHIOPODA. 



657 



the ribs carry a larger or smaller number of longer or shorter tubular 

 spines, which are especially abundant upon the auricular expansions. 

 The species of Producta range from the Devonian to the Permian, 

 but the genus is essentially and especially characteristic of the Car- 

 boniferous period. 



The genus Strophalosia (fig. 508, b) ranges from the Devonian to the 

 Permian, and is distinguished from Producta chiefly by the fact that the 



Fig. 506. — Dorsal and profile views of Producta 

 semireticulata. Carboniferous. 



Fig. 507. — Dorsal aspect of Producta 

 horrida. Permian. (After King.) 



valves are not edentulous, but are articulated by teeth and sockets ; 

 while each valve has a distinct hinge-area, and the ventral valve has a 

 foramen covered with a deltidium. Aulosteges, again (fig. 508, a), from 

 the Permian, has no teeth or dental sockets — in this respect agreeing 

 with Producta — but the ventral valve has a wide hinge-area, pierced by 

 a foramen, which is covered by a convex pseudo-deltidium. Aulosteges 

 is probably only a sub-genus of Strophalosia, but the shell is free, whereas 



Fig. 508. — A, Aulosteges Wangenheimii — Permian, showing the hinge-area and deltidium ;_b, 

 Strophalosia Gold/ussi, viewed dorsally— Permian ; c, Productella onusta— Devonian— interior 

 of the dorsal valve, showing the cardinal process (c), the muscular scars (;/z), and the reniform 

 vascular impressions (v). (After Davidson and Hall.) 



in the latter the shell is attached by the beak of the ventral valve. Lastly, 

 in the Devonian Productella (fig. 508, C) the valves are articulated by 

 teeth and sockets, and a hinge-area is present in both valves, but the 

 latter is narrow and linear, and the ventral valve is extremely convex 

 and gibbous. 



In the genus Chonetes (fig. 509) the shell is concavo-convex, trans- 

 vol. 1. 2 T 



