EL'ASIP'ODA. See HOLOTIILTRIAN. ELAS1VIOBRANCHII, ( Nen Lat. nom. pi., from Gk. Aaauds, elasmos, metal plate fipci7xta, branchia, gills), or ClIONDROp. TERYGII. A group of fishes whose skeleton con sists of cartilage and connective tissue alone. there being no distinct bones present, eompris. ing only the sharks, rays, and chimaeras. The cranium is composed of a single piece, without sutures and separate parts as in the higher verte brates. The skin is in nearly all cases provided with placoid scales. The males in the recent forms have the ventral fin modified into copula tory organs known as 'elaspers."the caudal fin is heterocereal. The gill-slits, five to seven in number, open directly to the exterior (except in the chimaeras), and the gill-filaments are fastened for their whole length to the inter branehial partitions. The intestine has a spiral valve, a fold of its wall into the interior ar ranged in n spiral fashion. No swimming blad der is present. The eggs are large and few in number. and are fertilized inside the body of the mother, where. in many eases, they undergo
development. The two main groups, tutmely, the which minimises the sharks and rays, and the Thdoeep/m/i, or ehinneras, include. besides the many living. many extinet forms, our knowledge of which (the skeleton being carti laginous and therefore not preserved) is derived front fossilized teeth and dermal sealer. Fossil remains of the Selaeliii first appear in the Upper Silurian. and throughout the Paleozoic times are very abundant. The lloloeephali appear first in the Lower Jurassic% a few probable re mains flaying been found in the Upper Silurian. It is anion: the elasmobranclis that we find the most primitive type of fishes. One of these more generalized primitive groups, the Cestracionts, abundant in Paleozoic times, is repre sented to-day by two or three species of Port Jackson sharks (q.v.). None of the elasmo branchs are very small. and ant lug them are some of the largest of aquatic vertebrates. There are about 300 living species, all marine.