Jordan and Ever man n. fishes of North America. ]."> 



SUBCLASSES OF PISCES. 



.1. Skull without system of membrane boues (opercles, etc.). 



6. Suspensorium of the mandible present. Gills not free, being attached by tho outer 

 margin to the skin ; egi;s lew and large, impregnated and sometimes developed inter- 

 nally, covered with a thick leathery skin when developed externally ; embryo with 

 deciduous external gills ; no membrane bones about the head; upper jaw formed of 

 palatine and pterygoid elements without maxillary or premaxillary; skeleton car- 

 tilaginous ; skull without sutures ; tail hcterocercal ; ventral fins abdominal ; male 

 with large iutromittent organs or claspers attached to the ventral fins, these complex 

 in structure iu existing species ; shoulder girdle not attached to the skull ; skin 

 naked or covered with small rcuj,h scales, or spines, or bony bucklers; no air bladder; 

 arterial bulb with 3 series of valves ; optic nerves with a chiasma ; cerebral 

 hemispheres united ; gill openings slit-like, 5 to 7 in number ; jaws distinct from 

 the skull, joined to it by suspensory bones ; teeth distinct. (Sharks and Skates.) 



SELACHII. 



bb. Suspensorium of tho mandible wanting; no maxillary arch; ventral fins with claspers; 



gill opening single, leading to 4 gill slits; jaws coalescent with the skull; teeth 



united in the form of bony plates. (Chimaaras.) HOLOCEPHALI. 



d. Skull with a well-developed system of membrane bones (opercles, suborbital ring, etc.); 



gills free, attached to the gill arches by their bases only ; gill opening single on each 



side; eggs comparatively small and numerous; no claspers; a maxillary arch ; cerebral 



hemispheres not united. (True Fishes.) TELEOSTOMI. 



Subclass SELACHII.* 



(THE SHARKS AND SKATES.) 



This group, sufficiently denned above, includes among recent fishes, the 

 Sharks and the Rays, marine fishes mostly of large size, abounding in all 

 seas. The group is commonly divided into 2 orders, which are widely 

 different in appearance, so far as the extremes are concerned, but which 

 pass into each other by insensible gradations. 



We begin the group with the archaic type, the order or suborder Diplos- 

 pondyli, proceeding thence from the more generalized sharks to the special- 

 ized skates. The true sharks form an almost perfect gradation into tiie 

 skates, but there are no forms extant which connect the Dlplospondyli 

 with modern sharks. (ai-^a^o^, shark; the word originally meaning 

 cartilage.) 



ORDERS OF SELACHII. 



a. Gill openings 6 or 7 ; dorsal fin single ; vertebral column imperfectly segmented, each seg- 

 ment being equivalent to 2 vertebrae, and bearing 2 neural aches ; anal fin present. 



DlPLOSPONDYLI, D. 



aa. Gill openings 5 ; vertebral column well segmented, each segment forming a neural 



arch and 1 centrum. 



b. Vertebrae each with the internal calcareous lamellae radiating from the central ring ; 



anal fin present. ASTEUOSPONDYLI, E. 



bb. Vertebra? with the internal calcareous lamellae not radiating, but arranged in one or 



more concentric circles or series around the central ring ; no anal fin ; palato- 



quadrate arch not articulated to the skull. 



c. Gill openings lateral ; dorsal fins 2. CYCLOSPONDYLI, F. 



cc. Gill openings ventral ; dorsal fins small and posterior, or wanting ; body and pec- 

 toral fins forming a depressed disk. BATOIDEI, G. 



* Also frequently known as Elasmobranchii, Plagioslomala, Chondroplerygia, or Placoidei. In Prof. 

 Cope's system, the Selactiii constitute one of the 2 orders in this subclass and all the existing 

 sharks and skates are referred to it. In the Selachii, as thus understood, the claspers are complex, 

 while in the extinct order of Ichthyotomi these organs are simple. 



