EIGHTH FAMILY--SPINACIDÆ. _No membrana nictitans. Two dorsal fins; no anal. Mouth but slightly arched; a long, deep, straight, oblique groove on each side of the mouth. Spiracles present; gill-openings narrow. Pectoral fins not notched at their origin._ The oldest representative of this family (_Palæospinax_) occurs at Lyme Regis; its skin is granular; each dorsal fin possesses a spine; the teeth in the jaws are dissimilar--the upper being multicuspid, longitudinally ribbed as in Hybodus, the lower smooth and tricuspid. _Drepanophorus_ and _Spinax primævus_ occur in Cretaceous formations of England and the Lebanon. CENTRINA.--Each dorsal fin with a strong spine. Trunk rather elevated, trihedral, with a fold of the skin running along each side of the belly. Teeth of the lower jaw erect, triangular, finely serrated; those of the upper slender, conical, forming a group in front of the jaw. Spiracles wide, behind the eye. One species, _Centrina salviani_, from the Mediterranean and neighbouring parts of the Atlantic; of small size. ACANTHIAS.--Each dorsal fin with a spine. Teeth equal in both jaws, rather small; their point is so much turned aside that the inner margin of the tooth forms the cutting edge. Spiracles rather wide, immediately behind the eye. The two species of “Spiny Dog-fishes,” _A. vulgaris_ and _A. blainvillii_, have a very remarkable distribution, being found in the temperate seas of the Northern and Southern Hemispheres, but not in the intermediate tropical zone. They are of small size, but occur at times in incredible numbers, 20,000 having been taken in one scene on the Cornish coast. They do much injury to the fishermen by cutting their lines and carrying off their hooks. CENTROPHORUS.--Each dorsal fin with a spine which, however, is sometimes so small as to be hidden below the skin. Mouth wide. Teeth of the lower jaw with the point more or less inclined backwards and outwards. Upper teeth erect, triangular, or narrow, lanceolate, with a single cusp. Spiracles wide, behind the eye.