282 Raymond — Common Devonian Brachiopods. 



Convexity of Valves. — In the nepionic stage both valves are 

 convex, but, when a length of about '5 mm is reached, the dorsal 

 valve becomes concave in front and follows very closely the 

 curvature of the other valve throughout succeeding stages. 

 Shells from 1 to 6 mm long are very slightly convex, sometimes 

 almost flat, but, as they grow older, the convexity increases 

 until in the gerontic stage they are almost hemispherical. 



Muscle Soars. — The migration of the muscles in this species 

 during the life history is most interesting. The normal form 

 of the scars in the ventral valve of the adult is shown in 

 Paleontology of New York, Yol. 4, Plate 15, figure 10, and 

 figure 2h, Plate 18, gives the interior of a dorsal valve at 

 the same stage. Figure 2h, Plate 18, and figure 11, Plate 15, 

 show the ventral and dorsal valves of a senile individual. 



In the smallest ventral valve in which the muscles have left 

 distinguishable impressions (4x4 mm ) the diductors have oval, 

 somewhat widely separated scars, between which are the two 

 small adductors, one on either side of the median line. The 

 diductors are bounded posteriorly by two ridges making a wide 

 angle with each other. The adductor scars have faint ridges 

 on either side and another ridge between them. In a little 

 later stage these three ridges become sharp and distinct. The 

 median one runs nearly to the beak, while the others remain 

 short, sharp, and rather high, curving outward. In the later 

 neanic stages these ridges arch over and join the* ridges which 

 bound the diductors. 



The two ridges bounding the posterior borders of the 

 diductors send off processes a short distance in front of the 

 hinge, which turn inward and run parallel for a little way. 

 They rise sharply from the floor of the valve, and overhang on 

 the side toward the median line. In the later neanic stages 

 the whole extent of the diductor impressions is bounded by a 

 low, sharp ridge that is later resorbed. 



During the adult stage the parallel portions of the two 

 ridges which bound the diductors are extended and strength- 

 ened, and the divergent portions resorbed. The median ridge 

 becomes stronger and rounded, and the two sharp ridges which 

 separated the adductors from the diductors disappear. An 

 almost square muscle scar is thus produced, which has less area 

 for attachment of the muscle, but is better located for a direct 

 pull on the cardinal process. 



In the dorsal valve the two pairs of adductors occupy a small 

 space in front of the cardinal process, and are usually bounded 

 by a low ridge. The outer pair, the posterior adductors, make 

 up most of the scar. They are small, roughly triangular, and 

 situated close to the front of the cardinal process. The ante- 

 riors are narrow, and are situated on a platform between, and 



