949 



CHAPTER XLVIII. 



CLASS PISCES— continued. 



Orders Chimeroidei and Dipnoi. 



Order III. Chimeroidei. — The Chimaeroids are marine fishes, 

 regarded by some writers as a suborder of Elasmobranchei ; but 

 although they resemble Sharks in external contour, in the presence 

 of " claspers " on the pelvic fins of the male, and in the structure 

 of the egg-capsules, yet they present such important differences as 

 to indicate the propriety of referring them to a distinct order. The 

 skeleton is entirely cartilaginous, and the vertebral column only 

 imperfectly segmented ; the notochord being surrounded by a series 

 of cartilaginous rings, which may be partly calcified. The skin of 

 the typical forms is usually quite naked in the adult, but in the 

 young there is a row of small dermal ossifications on the back. 

 The skull is movably articulated to the vertebral column, and has 

 the hyomandibular fused with the palatopterygoid bar, and the latter 

 firmly united to the cranium, with which the mandible consequently 

 articulates without the intervention of a separate suspensorium — ■ 

 this arrangement being termed autostylic. The gill-clefts are four in 

 number, and protected by a fold of skin containing a cartilaginous 

 gill-cover ; their communication with the exterior being effected by 

 a single aperture. The mouth is always terminal ; and in the recent 

 forms each jaw carries one pair of molariform teeth, respectively 

 attached to the palatopterygoid and Meckel's cartilage (mandible), 

 with the addition of a smaller anterior pair of vomerine cutting-teeth 

 in the upper jaw — all these teeth persisting throughout life. The 

 fins are similar in structure and position to those of the Sharks ; the 

 first dorsal always carrying a strong spine, which articulates with 

 the neural spines of the vertebrae, and is thus susceptible of motion. 

 In the absence of a swim-bladder the Chimeroids, again, agree with 

 the Elasmobranchs. There is a lateral line strengthened by carti- 

 laginous rings. From the absence of any membrane bones, the 



