BRACHIOPODA. 



211 



Chonetes are distributed in the Silurian, Devonian, and Car- 

 boniferous periods, but they are most abundant in the last 

 of these. 



In the genus Producta (fig. 1 60) the valves are not articu- 

 lated by any apparatus of teeth and sockets, and the shell 



Fig. 160. Producta, cora, Carboniferous, a Dorsal valve ; b Ventral valve. 

 (After Dawson.) 



would appear to have been free in the adult condition. The 

 shell is generally transversely elongated, and is auriculate, or 

 furnished with ear-like expansions. There is a straight hinge- 

 line, usually shorter than the width of the shell. The hinge- 

 area is rudimentary or wanting. The ventral valve is convex, 

 the dorsal concave, following the curve of the former. The 

 surface is ribbed or striated, and the ribs carry a greater or 

 less 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 

 Carboniferous period. 



FAM. VIII. CRANIAD/E. Animal fixed to submarine ob- 

 jects by the substance of the ventral valve. Arms fleshy and 



Fig. 161. Crania Ignabergensis, Cretaceous. 



spirally coiled. No hinge or articulating processes ; upper or 

 dorsal valve limpet-shaped (fig. 161). The family includes the 



