1918] Davidson: The Musculature of Heptauchus maculatus 167 



a long curved radial which comes close to the clasper. The ventral 

 fibers form the wall of the muscular sac. Some of these are inserted 

 along the opposite side of the same radial, wliile the others continue as 

 tendinous tissue and are inserted distally on the basal cartilage to 

 form the outer lip of the groove. 



(Compressor (cp.). The compressor has its origin laterally from 

 the beta cartilage (/8). Its direction is posterior and lateral and it is 

 inserted on the most posterior radial cartilage. 



Muscles of the Unpaired Fins 



The radials of the dorsal fin are about nineteen in number. Their 

 origin is in the connective tissue of the dorsal longitudinal muscles 

 and on the fin-skeleton as in the other radials described. They are 

 inserted in the connective tissue sheaths which continue distally into 

 the dermal fin-rays. 



In the anal fin, which is on the mid-ventral line and posterior to 

 the cloaca, the muscle is not differentiated into radials, but appears as 

 an individual mass. The origin is from the connective tissue of the 

 ventral longitudinal muscles and the fin-skeleton. The insertion is in 

 the connective tissue sheath similar to that of the radials of the dorsal 

 fin. 



The spinal column continues into the upper lobe of the caudal fin, 

 making the posterior body musculature the main part of the muscula- 

 ture of the caudal fin. The ventral musculature of the caudal fin is 

 similar to that of the anal. It extends from the anterior extremity 

 of the fin almost to the tip of the tail. Its origin is in the fascia of 

 the ventral body muscles and from the fin-skeleton. Its fibers are 

 inserted in the connective tissue covering, which is continuous witli 

 the dermal fin-rays. 



Musculature of the Trunk 

 Johannes Miiller in 1834 described the trunk musculature of elas- 

 mobranchs as divided by the lateral septum into a dorsal and a ventral 

 part. A. Schneider also distinguishes a dorsal and ventral division 

 but describes layer-formation and the presence of a rectus muscle. 

 Humphry (1872) working on Mustelus, found that this division into 

 dorsal and ventral parts separated by the lateral septum, existed in 

 all forms. Each of these parts he again divided into two parts, the 

 dorsal into the medio-dorsal and latero-dorsal and the ventral into 

 the latero-ventral and medio-ventral parts. Humphry describes the 



