BALL: MOLLUSCA AND BRACHIOPODA. 3G3 



S. solen {mediterranea Lamarck + togata Poli) the chondrophore has no anterior 

 or posterior rib, and the anterior exposure of the internal part of the ligament is 

 reduced to a narrow line directed obliquely backward to the upper anterior angle 

 of the posterior adductor scar. 



In S. velum Say, the most common species of New England, we find the liga- 

 ment has become wholly opisthodetic, no part of it appears in advance of the 

 beaks, and there is no exposure in front of the nymphs (or chondrophores) of 

 the ligament on the interior surface of the shell. The chondrophore in each 

 valve has an anterior and a posterior prop, neither being produced into a rib, and 

 the posterior adductor scar is only partly included in the space between the two 

 props, the posterior prop toucliing the middle of the dorsal edge of the scar. In 

 S. borealis Totten, the same arrangement prevails, but the props are so strong 

 and the nymph so projecting that a small cave under the nymph is created. On 

 the anterior edge of the nyn)ph is a small, narrow, elongate, muscular scar which 

 may belong to one of the pedal retractors. S. panamensis Dall agrees with 

 S. velum, except that the anterior prop is produced along the anterior edge of the 

 adductor scar, as a feeble ridge. 



In *S. agassizii Dall there are no supports to the chondrophore and no interior 

 exposure of the ligament, wliich is opisthodetic and wholly external. As far as 

 can be judged from the material at hand, there seems to have been a series of 

 muscular attachments for a considerable distance along the dorsal margin of the 

 valves in front of the supposed pedal protractor scar above referred to. At least 

 there are strong radial striations resembling the scar of adhesion of a muscle. 

 S. patagonica E. A. Smith (Challenger Report, 1885) has a hinge apparently 

 similar to that of S. agassizii, but in the unique specimen a thickening along the 

 dorsal side, which I strongly suspect to be pathological. If this suspicion proves 

 correct, it is probable that S. macrodactyla, which is of four years later date, will 

 prove to be synonymous. 



The groups into which the species fall, according to their characters, are as 

 follows : 



I. Ligament aniphidetic, chiefly internal. 



1. Subgenus Solemta s. s. Ligament exposed internally, in advance of the chon- 

 drophore. 



A. rroximal part of the chondropliore prolonged as a thickened ridge part 



way across tlie interior of the valve 5. australis Lam. 



B. Base of the chondrophore divided, anterior part extended as a narrow 



ridge; posterior part forming a tliickened prop to the chondrophore. 



."?. parkinsonii Gray. 



C. Chondrophore thickened, without props or extended rib. 6\ solen v. Salis. 



n. Ligament opisthodetic, internal. 



2 Subgenus Petrasma Dall. Ligament not exjiosed internally in front of the 

 chondropliore. 



