130 SUB-CLASS ELASMOBRANCHII. 



In the Holocephali the autostyUc arrangement (p. 63) prevails, 

 the palato-quadrate bar being continuous with the skull. 



The branchial arches are typically and usually segmented 

 into four pieces on each side and a median piece, the basibran- 

 chial or copula. The dorsal of the four pieces is called the 

 pharyngobranchial, the second the epibranchial, the third the 

 ceratobranchial, while the ventral and smallest piece is called 

 the hypobranchial. 



The hypo- and basi-branchial pieces are somewhat variable, the hypo- 

 branchial of the anterior arches being often undeveloped (some Raji) and 

 the basibranchial being often absent from the anterior arches. In the 

 latter case the last basibranchial (cardiobranchial) which is always larger 

 than the others, is much developed and has several arches attached to it. 

 The hypobranchial of the first arch is frequently attached to the basihyal. 

 The last arch is always smaller than the others ; its pharyngobranchial 

 is fused with the pharyngobranchial of the preceding arch and its hypo- 

 branchial is not developed. In Raji the last arch articulates with the 

 shoulder girdle. In Squali the dorsal elements are not closely attached 

 to the vertebral column, but in Raji there is a firm attachment. All the 

 branchial arches except the last bear branchial rays on the epi- and cerato- 

 branchial segments ; and in most Squali cartilaginous rods called extra- 

 branchials are placed close beneath the skin near some of the external 

 branchial apertures ; they are absent in Raji. 



The supporting structures of the median fins are, as in the 

 case of the paired fins, of two kinds ; the cartilaginous somactids 

 and the horny dermotrichia. In the dorsal fins, in the 

 dorsal part of the caudal, and in the anal fin the dermotrichia 

 are carried by somactids which are usually imbedded in the 

 muscles and not attached to the neural or haemal spines. In 

 the ventral lobe of the caudal fin there are no somactids and the 

 dermotrichia are carried by the haemal spines. 



The pectoral girdle consists of two dorso-ventrally directed 

 cartilaginous rods, placed one on each side of the body, and 

 each divided by the surface (glenoid) for the articulation of 

 the skeleton of the pectoral fin into a dorsal portion, the scapula, 

 and a ventral portion, the coracoid. The two coracoids are 

 continuous with one another ventrally, beneath the pericardium, 

 but the scapulas end freely dorsally, except in Raji, in which 

 the dorsal portion, sometimes partly marked off as a supra- 

 scapula, is attached to the anterior vertebral plate (p. 126) of the 

 spinal column. 



The skeleton of the pectoral fin consists of a number of 



