Fishes of the Western North Atlantic 63 



number of branchial lamellae that probably aid in the oxygenation of blood to the eyes and 



brain.* 



Key to Subclasses 



I a. 5 to 7 pairs of gills and 5 to 7 pairs of gill clefts, each of the latter opening separately 

 to exterior} dorsal fin or fins, and spines if present, rigid, not erectile; skin with or 

 without dermal denticles; teeth numerous; upper jaw or palatoquadrate cartilage not 

 fused to cranium, although it may be locally attached to it; rostral cartilage fused to 

 cranium; vertebral centra more or less clearly differentiated, and the notochord more 

 or less constricted segmentally; at least some of vertebrae of trunk region with articu- 

 lated transverse ribs; the 2 halves of pelvic girdle fused into a single bar; anus and 

 urogenital canals discharge into a common cloaca; males withou*. prepelvic or frontal 

 tenacula. Elasmobranchii; Sharks, Skates, Rays, p. 63. 



lb. Only 4 pairs of gills and 4 pairs of gill clefts, with only I opening to the exterior on 

 each side of head;* dorsal fin and spine erectile; skin in adult naked, without dermal 

 denticles; teeth represented by 6 pair" of grinding plates; upper jaw or palatoquad- 

 rate cartilage fused with cranium; rostral cartilages articulated to cranium, not fused; 

 no vertebral centra, and the notochord not constricted segmentally; ribs lacking; the 2 

 halves of pelvic girdle separate; no cloaca, the urogenital aperture being distinct from 

 anus and posterior to it; males with an erectile prepelvic tenaculum, and usually with a 

 frontal tenaculum on the head also." Holocephali; Chimaeroids. 



Subclass ELASMOBRANCHII 



Sharks, Skates, Rays 



Characters. Five to 7 pairs of gill clefts, all opening separately to exterior and not 

 covered by an opercular fold of skin; dorsal fin or fins, and fin spines if present, rigid, not 

 erectile; spiracle present or absent; skin armed with numerous placoid scales or "dermal 

 denticles"; teeth numerous and in several series; no frontal or prepelvic tenacula in males; 

 notochord more or less constricted segmentally, persisting only between the vertebrae in 

 many cases; vertebral centra more or less well developed; at least some of vertebrae of 

 trunk region with short articulated ribs; upper jaw or palatoquadrate cartilage not fused 

 to cranium, although it may be firmly attached to the latter by i or 2 articular surfaces of 

 limited extent; lower jaw, or Meckel's cartilage, articulated to upper jaw, and as a rule 

 attached also to hyomandibular arch, which thus takes part ia the suspension of jaws;' 

 rostral cartilages ( 1-3) fused to cranium; the 2 halves of pelvic girdle fused into a single 

 bar; inner margin of each pelvic fin in males modified to form a copulating organ or 

 "clasper" grooved for the passage of the sperm and supported by an axial cartilage, the 



3. Among the skates, the water that reaches the gills is inhaled through the spiracles, at least for the most part. 



+. The true gill clefts open into a common branchial chamber (covered by an opercular fold of skin supported by 



cartilaginous rays) which opens to the exterior by a single secondary branchial aperture on each side. 

 5. The frontal tenaculum is lacking in the genus Harriotta, 

 I. Among notidanoids this arch is much reduced and has no attachment to the lower jaw. 



