78 STUDIES Oi> THE ELASMOBR.i.NCH SKELETON, 



this degeneration), in Hypnos and Trygonorhina the hyoid, now- 

 become quite similar to the succeeding arches, is connected with 

 the skull only through the epibranchial of the first branchial arch. 

 Related to the hyo-mandibular is the spiracular cartilage, a thin 

 usually four-cornered plate supporting the wall of the spiracle. 



The lower or distal portion of the hyoid arch consists of two 

 lateral pieces on each side, and of a mesial cartilage or copula. The 

 lateral pieces (as well as the hyomandibular) may bear rays similar 

 to those of the branchial arches ; the mesial piece may be a broad 

 plate or a narrow band of cartilage, or, as, in many Batoidei, may 

 become altogether aborted. 



The internal branchial arches, which support the gill-pouches, 

 are always five in number, except in Hexanchus, which has 

 six, and Heptanchus which has seven. Each branchial arch, 

 when typically developed, consists of a dorsal basal cartilage 

 usually thin and leaf-like, and lying free close to the ventral 

 aspect of the spinal column, sometimes styliform and articulating 

 with the spinal column by a distinct joint (some Rays) ; of two 

 mesial cartilages, a dorsal and a ventral, usually with deep grooves 

 on their inner surface for the insertion of muscle ; of a copulare, 

 and of a mesial ventral copula. In most, however, the copulpe 

 become greatly reduced and may form a single basibranchial plate 

 as in the Rays ; in Myllobatis and Trygon the copularia are 

 likewise amalgamated with this basal-plate. The fifth arch has 

 no copulare and no basal — its dorsal mesial articulating with the 

 basal of the fourth: very often it is connected by articulation 

 with the pectoral arch. 



The outer branchial arches, which are a series of cartilages 

 bounding the branchial apertures and situated at the extremity of 

 the branchial rays, are more rudimentary in Hexanchus and 

 Heptanchus than in other Sharks ; they are best developed in 

 Cestracion. They are absent in the Rays, though Gegenbaur found 

 rudiments of them in Rhynchobatus and Trygon. 



The -pectoral arch is a stout cartilage, the lateral portions of 

 which are curved backwards and inwards towards the vertebral 

 column, with which they may articulate (Rays). The ventral 



