138 PROCEEDINGS OF THE AMERICAN ACADEMY. 



b. Abnormal rays are found in all three collections and not more fre- 

 quently in one valve than the other. Not all rays are equally apt to be 

 affected, but only those near the middle or near the extreme margin. It 

 is especially the two rays at the ends of the series and five on each side 

 of the middle that are affected. The following cases may be cited. 



Type 1. An additional (single) ridge appears on the side of a normal 

 double ridge* (No. 15821). The extra ridge is of ontogenetioally recent 

 origin and cannot be traced to the young shell, but is, as it were, budded off 

 from the side of the adjacent normal ridge. '« 



Type 2. An additional (single) ridge is interpolated midway between two 

 normal double ridges (No. 14553). In the case given there is only a ridge 

 on the inside of the valve and no corresponding groove on the outer side. 



Type 3. The lateral ridge or one that occupies the middle of the furrow 

 shows various gradations from a single ridge to a more or less completely 

 double one (Xos. 13665, 16258). The groove showing the extra ridge lies in 

 a ray that was originally typical but suffered an injury when 20 mm. in 

 diameter. Since then there has been a tendance toward bifurcation of the 

 ray (No. 8140). Iu No. 13665 the evidence of injury is more doubtful ; but 

 the tendency to double began at a corresponding age of the mollusc. 



Type 4. Two narrow but normally double ridges are in contact, or the 

 groove between them is abnormally narrow (Nos. 13594, 13759, 14034). In 

 No. 13794 an injury to the edge of the shell and mantle occurred when the 

 pecten was of 10 mm. diameter, and twelve rays were laid down as a more or 

 less unconformable continuation of the young rays. In the readjustment of 

 new rays to each other two rays come to lie close together. A similar acci- 

 dent had occurred in No. 13759, but in No. 14034 a groove has been plainly 

 interpolated. 



Type 5. A lateral or median interpolated ridge may become obsolescent, 

 and a groove of unusual width result (Nos. 13555, 14610, 13912, 13760, 

 15691). In No. 13555 three grooves arose, after an accident (?), from four, 

 and one of them gradually broadened preparatory to division. Nos. 13912, 

 14610, 13760, and 15691 have had a similar history. 



To sum up, the abnormal rays are induced by abnormal conditions, as 

 follows : (1) A rib may differentiate off from one of the main ridges as an 

 independent ray ; (2) a rib may widen and divide into two equal ribs ; 

 (3) a rib may arise late in life independent of other ribs and gradually 

 gain a size coordinate with the others ; (4) a ridge may appear on the 

 inside of a valve without a corresponding groove on the outside (a condi- 

 tion that is normal in the genus Amussium) ; (5) after an injury, or when 



* A rib, as seen from the inside face of the shell, has square, angular edges 

 which usually are elevated as ridges. There is such a ridge running along each 

 edge of the rib ; i. e , the complete normal rib has a double ridge. 



