4. SCYMNID^. 
13 
and skates differ much from each 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- 
dei Giinther, Aiii, 353-433.) (Latin, sqitalus, a shark, from the Greek 
yahoi:, a shark, which comes from yaUrj, a weasel, or other musteline 
animal, from resemblances in habits.) 
ANALYSIS OF THE FAMILIES OF SQUALI. 
* Pectoral fins moderate, without deep notch at the base in front, 
t Anal fin absent. 
a. Dorsal fins without spine Scymnid.®, 4. 
aa. Dorsal fins each preceded by a stout sijine (rarely concealed in the skin). 
Spinacid^, 5. 
tt Anal fin present. 
5. Dorsal fins two. 
c. Dorsal fins both without spine. 
d. First dorsal fin behind the ventrals Ginglymostomatid.e, 6. 
dd. First dorsal entirely in advance of the ventrals. 
e. Caudal fin 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 developed, forming less than one-third of the total 
length ; eyes with nictitating membranes. 
7i. Head normally formed Galeorhinid^, 7. 
hJi. Head hammer-shaped or kidney-shaped by the extension of its 
sides SPHYRNID.E, 8. 
gg. Tail exceedinglj'^ long, forming about half the total length ; eyes 
without nictitating membranes ALOPiiDiE, 9. 
ff. Last gill-opening entirely in front of the pectoral. CARCiiARiir).E, 10. 
ee. Caudal fin lunate ; caudal peduncle ■with a keel on each side, 
i. Last gill-opening entirely in front of ventrals. 
j. Gill-openings moderate; teeth well developed. Lamnii).e, 11. 
jj. Gill-openings very large, nearly meeting under the throat; 
teeth small CETORniNiD.E, 12. 
a. Last gill-opening above the base of the pectoral. 
Rhinodontid^, 13 
ec. Dorsal fins each armed with a stout spine Heterodontid^, 14 
hb. Dorsal fin single; the first dorsal obsolete ; gill-openings 6 or 7. 
Hexanchid^, 15. 
** Pectoral fins very large, expanded at the base in front, this expansion separated 
from the neck by a deeii notch ; no anal fin . Squatinid^, 16. 
Super-family SCYMNOIDEA. 
{The Scymnoid Sharis.) 
Family IV.— SCYMNID.^. 
{The Sleeper Sharis.) 
Sharks with two dorsal fins, both icithout spine.) and no anal Jin; fins 
all small ; gill-openings small, entmely in advance of pectorals ; mouth 
