Suborder Cyclospondyli—In 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 Ichthyotomt, 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.
