202 The True Sharks 
of this group is probably to be found in or near the Cestractontes, 
as the strong dorsal spines of the Squalide resemble those 
of the Heterodontide. 
Suborder Cyclospondyli—tIn this group the vertebre have 
the calcareous lamelle arranged in a single ring about the cen- 
tral axis. The anal fin, as in all the tectospondylous sharks 
and rays, is wanting. In all the asterospondylous sharks, 
as in the Ichthyotomi, Acanthodet, and Chimeras, this fin is 
present. It is present in almost all of the bony fishes. All 
the species have spiracles, and in all are two dorsal fins. None 
have the nictitating membrane, and in all the eggs are hatched 
internally. Within the group there is considerable variety 
of form and structure. As above stated, we have a perfect 
gradation among Tectospondyli from true sharks, with the 
gill-openings lateral, to rays, which have the gill-opening on 
the ventral side, the great expansion of the pectoral fins, a 
character of relatively recent acquisition, having crowded the 
gill-openings from their usual position. 
Family Squalide.—The largest and most primitive family 
of Cyclospondyli is that of the Squalide, collectively known as 
dogfishes or skittle-dogs. In the Squalide each dorsal fin has 
a stout spine in front, the caudal is bent upward and not keeled, 
and the teeth are small and varied in form, usually not all alike 
in the same jaw. 
The genus Squalus includes the dogfishes, small, greedy 
sharks abundant in almost all cool seas and in some tropical 
Fic. 144 —Dogfish, Squalus acanthias L. Gloucester, Mass. 
waters. They are known by the stout spines in the dorsal fins 
and by their sharp, squarish cutting teeth. They are largely 
sought by fishermen for the oil in their livers, which is used to 
adulterate better oils. Sometimes 20,000 have been taken in one 
