156 SUB-CLASS ELASMOBRANCHII. 



is on eacli side in the male a peculiar structure consisting of 

 a plate carrying teeth and sunk in a pit just in front of the 

 pelvic fin. 



The head of the male is provided with an erectile hook-like 

 process projecting forwards over a groove and armed on its lower 

 surface with small spines. The skin is usually naked except 

 in the young, in which small placoid spines are found principally 

 in a double row on the back. The lateral line may be an open 

 groove {Chimaera) or a closed canal {Callorhynchus). In 

 Chimaera the lips of the groove are approximated on the 

 head (Fig. 42), but remain apart at intervals giving the appear- 

 ance of openings. AmpuUary canals are present as in Plagio- 

 stomes. The eyes are without lids. 



Thenotochordal sheath is thick, cartilaginous and unsegmented. 

 It contains in Chimaera numerous calcified rings (four or five to 

 each segment). The arch tissue is segmented, except in the 

 front and in the whip-like tail, and does not meet round the 

 notochord sheath, except again at the front end and in the tail. 

 The neural arches consist of two pairs of pieces and a dorsal 

 piece for each pair of spinal nerves. The neural spine of the 

 anterior fused arch tissue is large and carries the anterior dorsal 

 fin, the basals of which are fused into one piece. 



The skull is autostylie (p. 63). has rostral continuations and is 

 without the prefrontal fontanelle. It has a well marked mem- 

 branous interorbital septum, which is placed dorsal to the brain. 

 It articulates with the vertebral column by two condyles. The 

 auditory capsule is incomplete internally so that the space for 

 the membranous labyrinth is open to the cranial cavity. There 

 are three pairs of labial cartilages and the hyoid arch which 

 carries branchial rays is attached by ligament to the skull. 

 There are five branchial arches. The hyoid arch carries a 

 demibranch (uniserial), the first three branchials each have a 

 holobranch (biserial), and the fourth branchial carries a demi- 

 branch. There is no gill-cleft between the fourth and fifth 

 branchial arches. The gill filaments are attached as in Plagio- 

 stomes and do not project. 



The paired fins and their girdles are formed on the Plagiostome 

 type, except that the two halves of the pelvic girdle are united 

 only by ligament. 



The teeth are large and few in number consisting of strong 



