The True Sharks 545 



of this group is probably to be found in or near the Cestraciontes, 

 as the strong dorsal spines of the Squalidce resemble those 

 of the Heterodontida. 



Suborder Cyclospondyli. In this group the vertebrae have 

 the calcareous lamellae 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, Acanthodei, and Chim&ras, 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 Squalidae. The largest and most primitive family 

 of Cyclospondyli is that of the Squalidcs, collectively known as 

 dogfishes or skittle-dogs. In the Squalidaz 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 



FIG. 338. 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 



