Three species, of which one (_G. arcticus_) is confined to the arctic and sub-arctic oceans. The others inhabit temperate and tropical seas, and all attain to a very large size. GALEUS.--Snout produced in the longitudinal axis of the body; teeth equal in both jaws, rather small, flat, triangular, oblique, serrated and with a notch. Spiracles small. No pit at the commencement of the caudal fin, which has a single notch on its lower margin. These are small sharks, commonly called “Tope.” The species found on the British coast is spread over nearly all the temperate and tropical seas, and is common in California and Tasmania. It lives on the bottom, and is very troublesome to fishermen by constantly taking away bait or driving away the fishes which they desire to catch. ZYGÆNA.--The anterior part of the head is broad, flattened, and produced into a lobe on each side, the extremity of which is occupied by the eye. Caudal fin with a single notch at its lower margin. A pit at the root of the caudal fin. Spiracles none. Nostrils situated on the front edge of the head. The “Hammerheads,” or Hammerheaded Sharks, have a dentition very similar to that of _Carcharias_, and although they do not attain to the same large size, they belong to the most formidable fishes of the ocean. The peculiar form of their head is quite unique among fishes; young examples have the lateral extension of the skull much less developed than adults. Five species are known, which are most abundant in the tropics. By far the most common is _Zygæna malleus_, which occurs in nearly all tropical and sub-tropical seas. Specimens of this species may be often seen ascending from the clear blue depths of the ocean like a great cloud. Cantor found in a female, nearly 11 feet long, thirty-seven embryons.--Hammerheads have lived from the cretaceous epoch. MUSTELUS.--The second dorsal fin is not much smaller than the first. No pit at the root of the caudal, which is without distinct lower lobe. Snout produced in the longitudinal axis of the body. Spiracles small, behind the eyes. Teeth small, numerous, similar in both jaws, obtuse, or with very indistinct cusps, arranged like pavement. The “Hounds” are small Sharks, abundant on the coasts of all the temperate and tropical seas; two of the five species known occur on the coasts of Europe, viz. _M. lævis_ and _M. vulgaris_. Closely allied as these two species are, they yet show a most singular difference, viz. that a placenta is developed in the uterus for the attachment of the embryo in _M. lævis_ (the Γαλεὁς λεȋος of Aristotle, to whom this fact was already known); whilst the embryons of _M. vulgaris_ are developed without such placenta (see _J. Müller_, “Abhandl. Ak. Wiss.” Berl. 1840). The Hounds are bottom fish, which feed principally on shells, crustaceans, and decomposing animal substances. * * * * *