64 THE ELASMOBRANCH FISHES 



posteriorly to the middle portion of the hyomandibula {Torpedo, fig. 63). 

 Such a union results from the method of growth of the hyomandibula. A proc- 

 ess extends from the anterior angle of the upper segment which, in the adult 

 Torpedo, forms the suspensorium or hyomandibula. This part suspends the 

 mandibular arch while the lower segment is attached to the posterior part of 

 the hyomandibula (fig. 69). In some of the other rays the lower segment may 

 not be attached at all to the hyomandibula, but may be united with the poste- 



c.b. 



Fig. T. 



Fig'. 71 



Fig. 71. First branchial arch, Heterodontus francisci. (Duncan Dunning, del.) 



Fig. 72. Fourth and fifth branchial arches, Heterodontus francisci. (Duncan Dunning, del.) 



h.r., branchial ray; cl>., ceratobranchial ; eJ}., epibranchial ; ex.l}., extrabranchial carti- 

 lage; ph., pharyngobranchial. 



rior part of the cranium (Urolophus) . In a still more specialized form it may 

 have no union either with the hyomandibula or with the cranium, but may be 

 bound to the tip of the first branchial (third visceral) arch, as in Rhinohatis 

 and Myliohatis. In some such occurrences the lower arch may be further 

 segmented. 



The branchial arches in general are typically made up of four segments (fig. 

 71, Heterodontus) which from dorsal to ventral, as was given for Heptanchiis, 

 are: (1) the pharyngobranchial {j)h.), (2) the epibranchial (e.h.), (3) the 

 ceratobranchial (ch.), and (4) the hypobranchial {hh., fig. 73) segments. 



The pharyngobranchials are usually flattened cartilages which lie dorsal to 

 the pharynx. In .sharks they are usually attached bj^ strong connective tissues 

 (ligaments) to the roof of the pharynx or to the sides of the spinal column 

 but not to the pharyngobranchials of the opposite side, as is the first in Hep- 

 tanchus and Scyllium. In the rays, the pharyngobranchial segment of the first 

 branchial arch, as we have said, may join the cranium {Rhinohatis, Trygon). 



