14 BULLETIN 100, UNITED STATES NATIONAL MUSEUM 



a^ Dorsal fins without spines. 



&\ Eye without nictitating membrane. 

 &. Caudal moderate or small, less than half of entire fish. 

 6}. Last gill opening above pectoral base. 



e^. First dorsal lobate, median, or postmedian. 



f. ScYT.ioRHiNOiDEi. Caudal peduncle without keel each side ; teeth 



raptorial; nasoral grooves present or absent; spiracles present; 2 



dorsals (1 in Pentanchus) ; caudal axis little raised, subcaudal 



not produced. 



g\ Caudal short, axis not raised ; nasal cirri absent or present ; 



nasoral grooves absent or rudimentary Scyliorhinidae 



f/l Caudal long, axis little raised ; nasoral grooves and nasoral cirri 



present Orectolobidae 



f. IsuEoiDEo:. Caudal peduncle with lateral keel each side ; caudal 



lunate or subcaudal well developed ; caudal peduncle often with 



keel on each side ; snout pointed, variously long or short ; no 



nasoral groove ; spiracles present, rarely absent. 



7i\ Gills without strainers; teeth large, subulate or triangular, 



entire or serrate Isuridae 



?^^ Gills with strainers ; teeth minute ; size enormous. 

 i\ Snout subcouie ; eyes above forward end of mouth ; gill 



openings extend nearly around neck Halsydridae 



il Snout broad ; eyes near mouth angles ; gill openings mod- 

 erately wide, last above pectoral base — Rhincodontidae 

 e^. PsEUDOTEiAKOiDEi. First dorsal low, long keel, premedian. 



Pseudotriakidae 



d'^. Caechaboidei. Last gill opening entirely before pectoral base ; caudal 



moderate, axis low ; snout variously short to very long — Carchariidae 



c*. Aix>PioiDAE. Caudal longer than body, axis rather low; snout short, 



conic , Alopiidae 



l". Sphtenoidei. Nictitating membrane present; head attenuated, sometimes 

 expanded across interorbital region ; teeth various, compressed or triangu- 

 lar, sometimes in bands or pavements; nasoral grooves absent (except 

 Scylliogaleus) ; caudal axis slightly raised, subcaudal produced. 

 f. Head not expanded across orbital region. 



Galeorhinidae 



f. Head well expanded across orbital region, kidney-shaped 



to greatly hammer-shaped Sphyrnidae 



Family HETERODONTIDAE 



Body thick set anteriorly, somewhat trihedral, rather short. Head 

 thick, high, large, supraorbital ridges prominent. Snout projects 

 but little in front. Eyes lateral, small. Mouth narrow, terminal. 

 Upper lip with 7 lobes and fold on lower lip. Teeth alike in jaws, 

 small and conic in front, large molars behind. Nostrils with 2 thick 

 valves, inferior, nearly terminal, with grooves to mouth. Gill open- 

 ings 5, of which several above pectorals. Spiracle small, below eye. 

 Scales fine, carinate. Dorsals two, moderate, strong spine on each, 

 first opposite space between pectorals and ventrals, second before 



