598 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxvi. 



FAMILIES OF ASTEROSPONDYLI. 



I. Ckstraciontes. Palatoquadrate apparatus articulated to preorbital part of skull; 

 dorsal fius with .spines; liead short and blunt; teeth of differing forms in the 



same individual HeterodontidzE, III. 



II. (talei. Palatoquadrate apparatus not articulated with skull; no dorsal spines; 

 head more or less pointed in profile; teeth not differing widely in form in the 

 same individual. 

 a. First d( >rsal fin ( )ver or behind ventrals; spiracle present; no nictitating meml)rane. 

 }). Tail not 1)ent upward; nostrils not confluent with the mouth. 



r. Sharks oviparous Hcyliorhinid.k, IV. 



(T. Sharks ovoviviparous Hemiscvij.iid.k, V. 



aa. First «lorsal fin inserted more or less in advance of ventrals. 



d. First dorsal fin high, highest anteriorlj', its base wholly in front (if that 

 of ventrals. 

 ee. Caudal fin not lunate, its upper lobe two or more times length of lower, 

 with a notch below toward its tip; side of tail not keeled. 

 /. Last gill-opening above base of pectoral. 



(J. Tail moderately developed, forming less than one-third of the total 

 length; eyes with nictitating membranes. 



h. Head normally formed Carchariid^, VI. 



////. Head hammer-shaped or kidney-shaped by the extension of its 



sides SPHYRNIDiE, VII. 



[III. Tail exceedingly long, forming about one-half the total length; 



eyes without nictitating membrane Alopiid.e, VIII. 



ff. Last gill-opening entirely in front of pectoral; snout ending in a long 



flat blade Mitsukurinid.e, IX. 



ee. ('audal fin lunate; caudal peduncle with a keel on each side; size large. 

 /. Last gill-()pening entirely in front of pectorals. 

 ;'. (Jill-openings moderate; teeth large and sharp. .Lamnid.e, X. 

 jj. ( Jill-oi)enings very wide, nearly meeting under throat; teeth 

 very sniall and numerous; size enormous. 



Cetorhiniu.k, XI. 



ii. Last gill-opening above base of pectorals; teeth small; size 



large Rhinodontiij.e, XII. 



Family III. HETERODONTIDvE. 



CESTRAC^IONT SHARKS. 



Body elongate; obtusely trihedral, gradually tapering backward; i 

 head liigh, with the forehead declivous, and little prominent. Mouth I 

 rather nai'row, the upper lip divided into 7 lol)es, the lower witli a 

 fold; dentition similar in both jaws, small obtuse teeth in front, and 

 the hiteral teeth niolar-lilvo and enlarged. Nostrils confluent with the 

 mouth. (Jill-openings ^k Spiracles small. Dorsal tins 2, and each 

 provided with a strong spine. Caudal tin usually notched at tip. 

 ()vii)ar()us, the egg-cases very large, sul>conical, without tentacles, 

 and spii-ally twisted. Small sharks now inhabiting the Pacific Ocean. 

 Species supposed to belong to the same family are widely distributed 

 as fossils in the Mesozoic and earlier periods. 



