388 MANUAL OF ZOOLOGY SECT. 



sisting in the large spaces between the concave surfaces 

 of contiguous centra. In the Rays the anterior part of 

 the spinal column becomes converted into a continuous 

 solid mass the anterior vertebral plate (Fig. 217, a.v.p.). 

 Two main regions only are distinguishable in the spinal 

 column the precaudal and the caudal, the latter being 

 distinguished by the presence of inferior or haemal arches. 

 In the precaudal region short ribs may be developed, but 

 these are sometimes rudimentary or even entirely absent. 



The skull is an undivided mass of cartilage, hardened, in 

 many cases, by deposition of calcareous matter, but not 

 containing any separate bony elements. In all, the jaws are 

 connected with the skull through the intermediation of a 

 hyomandibular cartilage, or proximal element of the hyoid 

 arch : in the great majority this is the sole articulation of 

 the jaws with the skull posteriorly, and the skull is on that 

 account said to be hyostylic\ but in Hexanchus and 

 Heptanchus (Fig. 218) the upper jaw has a direct articula- 

 tion with the skull behind the orbit, and the arrangement is 

 termed amphistylic. There are always five pairs of 

 branchial arches, except in Hexanchus and Chlamy- 

 doselachus, which have six, and Heptanchus, in which there 

 are seven. 



The basal cartilages of the pectoral fin are typically three, 

 as in the Dog-fish, but there are sometimes four, and the 

 number may be reduced to two. There are usually two 

 such cartilages in the pelvic fin, and one alone may be 

 present. 



Electric organs organs in which electricity is formed and 

 stored up, to be discharged at the will of the Fish occur 

 in several Elasmobranchs. They are best developed in the 

 Electric Rays (Torpedo and Hypnos) in which they form a 

 pair of large masses running through the entire thickness of 



