CHIM^ERID^E. 285 



Order II--ELASMOBRANCHII. 



Skeleton cartilaginous : no cranial sutures. Rarely a single gill-opening 

 as the gills are attached by their outer edges to the skin, and an intervening 

 gill-opening exists between each : no gill-cover. Two or more series of valves at 

 the conus arteriosus. Optic nerves, united, but do not decussate, or only slightly 

 so. Body with vertical and paired fins, the posterior pair abdominal : caudal 

 with an elongated upper lobe. Intestines with a spiral valve. Male sex with 

 intromittent organs attached to the ventral fins. Ova large, fertilized, and in 

 some likewise developed, internally. Embryo with external deciduous gills. No 

 air-bladder. 



The fishes which compose this order, have been divided into two sub-orders, as 

 follows : — 



I. Holocephala — jaw and palate attached to the skull. One external gill- 

 opening. 



II. Plagiostomata — palatal apparatus detached from the skull, and from five 

 to seven gill-openings. 



Sub-Order I— HOLOCEPHALA. 



A single external gill-opening and a rudimentary gill-cover. Inside the single 

 gill cavity four gill-openings. The maxillary and palatal apparatus attached 

 to the skull. 



The living fishes appertaining to this sub-order are confined, so far as is known, 

 to the representatives of one family, the CbimseridaB, which has been generally 

 considered, to form a connecting link between the Ganoids and the sharks and rays. 

 For although externally possessing but a single gill-opening and a rudimentary 

 opercle, four orifices may be seen at the bottom of the gill-pouch. These orifices 

 correspond to the intervals between the gills, which are not free at their outer 

 extremities as described in the sturgeons and bony fishes, but are adherent to the 

 skin as observed more readily in the sharks and rays. Chiniasras possess three 

 complete biserial gills, also an accessory uniserial gill at the inner side of the 

 opercle, and a second attached to the fourth arch or inferior pharyngeal. In 

 the skin of the young are placoid granules along the middle of the back as 

 perceived in some of the rays. The ova are few but large, encased in a 

 horny covering, and impregnated within the oviduct, while the male possesses 

 claspers. In several chai'acters, in addition to those enumerated, they approach 

 the Ganoids, as, for instance, in the spine of the dorsal fin being articulated to the 

 neural apophysis of an anterior vertebra, and not simply implanted into the skin : 

 in the dental apparatus consisting, as in some Dipnoids, of plates and some cutting 

 teeth in the upper jaw. Skeleton notochordal and cranial cartilages continuous. 



Family I— CHDLERID^E. 



Body elongated. Mouth on the lower surface. Dental apparatus consisting 

 of two plates in the upper, and one in the lower jaw. No spiracle. Males with a 

 filament on the upper surface of the snout, also with claspers. Skin scaleless in 

 adults. 



This family consists of two genera: (1) CaUorhynclms, in which the snout ends 

 anteriorly in an elongated flap : and (2) (Jhirncera, destitute of such appendage. 



