BRACHIOPODA 



175 



Productus (fig. 78). Shell free, or fixed by spines, generally 

 transverse {i.e. broader than long) but sometimes elongated, often 

 produced into ' ears ' at the sides. Dorsal valve concave. Ventral 

 valve very convex, often sharply bent, sometimes with a median 

 sinus ; umbo large, incurved, not perforated. Hinge-line straight, 

 teeth absent or rudimentary. Area linear or absent. Surface orna- 

 mented with radiating ribs, crossed by concentric folds, especially in 

 the umbonal region. Tubular spines, especially in the region of the 

 umbo and ears. Muscular impressions strongly marked ; in the 

 ventral valve the adductors (a) are near the umbo, and in front of 

 them are the divaricators (r). A prominent cardinal process (J) on 

 the dorsal valve is continued as a median ridge in the interior. No 

 brachial skeleton. Devonian to Permian. Ex. P. semireticvlatus. 

 Carboniferous Limestone. 



Strophalosia. Shell similar to Productus in form ; attached 

 by umbo of ventral valve. A distinct area on each valve, with 

 a pseudo-deltidium ; the ventral area larger than the dorsal. 

 Ventral valve with two prominent teeth. Dorsal valve with a 

 prominent, bifid cardinal process. Surface of ventral (and sometimes 

 also the dorsal) valve covered with spines. Devonian to Permian. 

 Ex. S. excavata, Permian. 



A 



B 



Fig. 79. Chonetes from the Devonian. A, dorsal ; B, ventral valve. 

 a, adductor impressions; c, divaricators; t, teeth; v, vascular 

 impressions ; j, cardinal process. (From Woodward.) Enlarged. 



Chonetes (fig. 79). Shell transverse, semicircular, concavo- 

 convex, or sometimes plano-convex. Hinge-line straight, forming 

 the greatest width of the shell. Teeth strong. An area on each 

 valve ; dorsal area very narrow. Upper margin of area of ventral 

 valve with a row of hollow, diverging spines, which increase in 



