656 



MOLLUSCOIDEA. 



All the members of the Productida are Palaeozoic, the earliest 

 forms appearing in the Silurian, while the last are found in the Per- 

 mian deposits. 



In the genus Producta (or Productus) itself (figs. 505-507) the 

 valves are not articulated by teeth and sockets, and appear to have 

 been generally free in the adult condition. In some cases, how- 

 ever, where the shell is furnished with long spines, it may be sup- 

 posed that these structures served to moor the animal in the soft 

 ooze of the sea-bottom. In the curious Producta complectens, Mr R. 

 Etheridge, jun., has shown that the shell was firmly attached by the 



Fig. 505. — Structure of Producta gigantea, from the Carboniferous Limestone, a, Interior of 

 the dorsal valve ; b, Interior of the ventral valve, a portion of the projecting beak being removed ; 

 c, Ideal section of both valves, in place, in the middle line ; d, Hinge-line of the dorsal valve. 

 a a, Adductor impressions ; d d, Divaricator impressions (in the ideal fig. c the letters a and d 

 indicate the adductor and divaricator muscles respectively) ; z", Reniform vascular impressions ; 

 J>, Processes supporting the bases of the arms ; s, Hollows occupied by the spiral arms ; ca, 

 Cardinal process ; h, Hinge-line; do, Dorsal valve ; ve, Ventral valve. (After Woodward.) 



twisting of some of the spines of the ventral valve round the column 

 of a Crinoid. In the still more singular Producta firoboscidea, the 

 ventral valve is prolonged beyond the dorsal, and its edges are 

 rolled together and form an elongated tube, by which the shell was 

 attached to some foreign body. The shell in Producta is generally 

 transversely elongated in shape, and is " auriculate " or furnished 

 with ear-like expansions. There is a straight hinge-line, usually 

 shorter than the greatest width of the shell, the hinge-area being 

 linear or wanting. The ventral valve is convex, with a large, bent, 

 and imperforate beak — the dorsal valve being concave, and follow- 

 ing the curve of the former. The surface is ribbed or striated, and 



