4. SCYMNIDiE. 13 



and skates differ much from eacli other in appearance ; but intermediate 

 forms connect the two groups closely. The position of the gill-openings 

 is almost the only constantly diagnostic character. (Suborder Selachoi- 

 dci Giinther, viii, 353-4:33.) (Latin, squalus, a shark, from the Greek 

 yaXsd-:, a shark, which comes from ya?j7], a weasel, or other musteline 

 animal, from resemblances in habits.) 



ANALYSIS OF THE FAMILIES OF SQUALl. 



*" Pectoral fins moderate, without (loop notcli at the base iu front. 

 t Anal fin absent. 



a. Dorsal fins without spine SCYMNiDiE, 4. 



aa. Dorsal fins each preceded by a stout spine (rarely concealed iu the skin). 



Spixacid^, .'). 

 tt Anal fin present. 

 b. Dorsal fins two. 

 c. Dorsal fins both without spine. 



d. First dorsal tin behind the veutrals GiNGLYMOSTOM ATID^, 6. 



dd. First dorsal entirelj" in advance of the Tentrals. 

 e. Caudal lin not lunate, its upper lobe many times longer than the lower, 

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

 /. Last gill-opening above the base of the pectoral. 



g. Tail moderately develoi^ed, forming less than one-third of the total 

 length ; eyes with nictitating membranes. 



h. Head normally formed Galeorhinid^, 7. 



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



sides Sphyrxidje, 8. 



gg. Tail exceedingly long, forming about half the total length ; eyes 



without nictitating membranes Alopiid.e, 9. 



ff. Last gill-opening entirely in front of the pectoral. Carchariid/E, 10. 

 ee. Caudal fin lunate ; caudal peduncle with a keel on each side. 

 i. Last gill-opening entirely iu front of veutrals. 



j. Gill-openings moderate ; teeth well developed. Lamnid^e, 11. 

 jj. Gill-openings very large, nearly meeting under the throat; 



teeth small CEXORHixiDiE, 12. 



ii. Last gill-opening above the base of the pectoral. 



Rhinodontid^, 13 



cc. Dorsal fins each armed with a stout spine Heterodontid.e, 14 



hb. Dorsal fin single; the first dorsal obsolete ; gill-openings 6 or 7. 



HexanciiiDvE, 1,5. 



** Pectoral fins very large, expanded at the base iu front, this expansion S('])arated 



from the nock by a deep notcli ; no anal iiu.SQUATiNiD.E, IG. 



Super-family SCYMNOIDEA. 



{TJie Scymnoid SharJcs.) 



Family IV.— SCYMNID^. 



{The Sleeper Sharks.) 



Sharks with two dorsal fins, both icWiont sjnne, and no anal Jin ; fins 

 all small ; gill-openings small, entirely in advance of pectorals ; mouth 



