Refokt or the State Geologist 613 



Aulosteges, von Helmersen. 1 V I7. 

 (Plate 81, Pigs. l'J-ii. 



Shell productiform in genera] aspect, somewhal depressed, not. 

 attached. On the pedicle valve the cardinal area is prominently 

 developed, frequently somewhat distorted; the delthyrium is 

 closed by a convex delt id inm which is covered with little tuber- 

 cles or spinules. Cardinal teeth rudimentary or absent. In the 

 brachial valve the cardinal area is linear, the cardinal pr< 

 large, quadrifid ; its base being surrounded by a strong deltidial 

 callosity. Brachial ridges extending nearly to the anterior 

 margin and abruptly incurving. Otherwise the interior impres- 

 sions are as in Productus. Surface of both valves thickly set 

 with spines. 



Type, Avhsteges Wangenheimi, De Yerneuil. 



Distribution, Permian. 



Productus, Sowerby. 1812. 

 (Plate 22, Figs. 1-12.) 



Shell free, concavo-convex, the valves usually produced 

 anteriorly; outline semicircular, sometimes transversely elongate. 

 Pedicle-valve convex, sometimes geniculated, occasionally with 

 a median sinus. Cardinal extremities frequently auriculate. 

 Umbo inflated, with apex incurved. Hinge-line straight, cardinal 

 area and teeth absent or rudimentary. External surface usually 

 with more or less prominent radiating ribs which are crossed, 

 especially in the umbonal region, by concentric lines of wrinkles ; 

 rarely smooth or finely striated, often studded with spines vary- 

 ing in size and abundance. These spines are frequently scattered 

 irregularly over the surface, generally strongest and disposed 

 with greatest regularity upon the cardinal extremities, and some- 

 times occurring only in this region. They are hollow and appear to 

 have communicated with the interior cavity of the shell. On the 

 interior of the valve is a narrow median ridge, separating the two 

 dendritic impressions of the adductor muscles; outside and in 

 front of these are two broadly flabellate, longitudinally striated 

 scars of the diductor muscles. In the pallial region are some- 

 times found traces of spiral cavities, which were occupied by the 

 fleshy arms. 



165 



