BRACHIOPODA. 



457 



fissure, closed in the ventral valve by a pseudo-deltidium. 

 The upper edge of the hinge-area of the ventral valve is 

 furnished with a row of delicate, tubular spines. The species 

 of Chonetes are distributed in the Silurian, Devonian, and 

 Carboniferous periods, but they are most abundant in the 

 last of these. 



In the genus Producta (figs. 309, 310) the valves are not 

 articulated by any apparatus of teeth and sockets, and the 

 shell would appear to have been generally free in the adult 



Fig. 309. Dorsal and profile views of Producta, 

 semireticulata. Carboniferous. 



Fig. 310. Dorsal aspect of Producta 

 horrida. Permian. (After King ) 



condition. In some cases, however, where the shell is fur- 

 nished with long spines, we may believe that these structures 

 served to moor the animal in the soft ooze of the sea-bottom. 

 Mr K. Etheridge, jun., has also shown that in one species 

 of the genus, at any rate, the shell was firmly fixed to one 

 spot by the twisting of some of the spines of the ventral 

 valve round the column of a Crinoid. 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 rudi- 

 mentary 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 Car- 

 boniferous period. 



