Joniaii and Evcrmann.—I'isht's of North America. 15 



Subclasses of Pisces. 



n. Skull without (i^stem of membrane boues (ojiercles, etc.). 



h. Suspcnsorium of tho mandiblo preseut. Gills not free, beiug attached by the outer 

 margin to the skin ; eggs few and large, impregnated and sometimes developed inter- 

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

 deciduous external gills ; no membrane bones about tho head; ujiperjaw formed of 

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

 tilaginous ; skull w ithout sutures ; tail hetorocercal ; ventral fins abdominal ; male 

 with large intromitttut organs or claspers attached to the ventral fius, these complex 

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

 naked or covered with small rou^h scales, or spines, or bony bucklers; uoairbladder; 

 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. 



66. Susponsorium of the mandible wanting; no maxillary arch; ventral fins with claspers; 



gill opening single, leading to 4 gill slits; jaws coalesccnt with tho skull; teeth 



united in the form of bony plates. (Chimairas.) Holocephali. 



<!((. 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 defined above, includes among recent fishes, the 

 Sharks and the Kays, 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- 

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

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

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

 with modern sharks. {ai7Mxo(:, shark; the word originally meaning 



cartilage.) 



Orders of Selacuii. 



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

 ment being equivalent to 2 vertebra^, and bearing 2 neural aches ; anal tin present. 



DlI'LOSPONDVLI, D. 

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

 arch and 1 centrum. 

 6. Vertebra! each with the internal calcareous lamelho radiating from the central ring ; 

 anal fin present. Astekospondvli, E. 



66. Vertebra; with tho 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 sk\ill. 

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



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

 toral fius forming a depressed disk. Batoidei, G. 



* Also frequently known as Elasmobraiicldi, Plagiostomaia, Chondropten/gia, or Placoidei. lu Prof. 

 Cope's system, the SWdcAiV 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, 

 w hile in the extinct order of Ichlhijolomi these organs are simple. 



