106 STUDIES ON THE ELASMOBRANCH SKELETON, 



The arrangement of the rest of the branchial apparatus in Hypnos 

 is likewise peculiar. The copular elements of the hyoid and of the 

 first, second, and third branchial arches are united below on either 

 side to a thick longitudinal bar of cartilage which unites behind 

 to the basi-branchial plate, close to its fellow, but diverges in 

 front so as to leave a considerable triangular space, and ends in a 

 free thickened extremity some little distance in front of the hyoid 

 arch. Though these bars present no trace of longitudinal division 

 they seem to correspond to the copularia of the hyoidean and 

 three anterior branchial arches. The fourth and fifth branchial 

 arches unite dorsally, as very commonly occurs, the basal of the 

 fifth arch not being represented ; from their point of union a long 

 process passes inwards and backwards. The fifth arch has an 

 articulation with the shoulder girdle. 



Pectoral Fins. (Plate II., fig. 8.) 



The shoulder-girdle is distinguished by the slenderness of its 

 mesial ventral portion, which, as in Rays generally, is formed of 

 one continuous bar, not interrupted by any fibrous interval such as 

 occurs in Torpedo. It bears externally the lateral masses of 

 cartilage with which the pectoral fins articulate, and an upwardly 

 directed process of each lateral mass is movably articulated with a 

 dorsal bar of cai-tilage which runs inwards towards the middle 

 line of the dorsal surface. The dorsal ends of these two bars are 

 united by a flattened mesial cross-piece, which is placed well above 

 the vertebral column, not being connected with it save by fibrous 

 tissue. Such an entire absence of cartilaginous connection 

 between the pectoral arch and the spinal column is found in no 

 other Eay with the exception of Torpedo. (Vide Gegen. Unters. 

 II., 1, p. 81.) The three facets for the pectoral fin are borne on 

 three short processes. The propterygium is made up of no less 

 than nine joints which decrease in size distally, the whole having 

 much the appearance of a magnified fin-ray. The mesopterygium 

 is a small cartilage shorter than the propterygium, and is divided 

 into seven segments. Each of the rays of the fin is bifurcate at 

 the tip. 



