ORTHOCERAS, 6r-th6s16-riis (Neo-Lat.. from Gk. opfik, ort/tos, straight. correct -4- Kr dac. keras, horn). A genus of fossil nautiloid ceph alopods, characterized by straight conic whose surface may he either smooth or trans verhely striated. The interior of the shells is divided into chambers by simple transverse septa, and there is a median siphunele. The members of this genus, which range from the Silurian to the "Friassie periods, are of much importance as index fossils. The limits of the genus as de fined by the majority of writers have been so broad as to include almost all of the straight shelled Pa letszoic cephalopods, and the assem blage of species thus brought together was found by Hyatt and other investigators to contain specks that belong more properly in a number of different families and several new genera. All of the straight-shelled forms are now classed to gether in an artificial group as orthoceracones, in distinction to the curved shells or eyrtocera cones. The orthoceraeones are in general ante cedent to the cyrtoceracones in each family or race. They appear first in the Upper Cambrian and are very abundant in the Ordovician rocks, where Or! horrros Woo of the Black River lime stone. with a length of about ten feet, is the largest known form of this group of cephalopods. During the Ordovician the orthoceraeones gave rise to several derived genera of curved aml coiled shells, and to shells with peculiarly restricted apertures. Some of these forms gave rise to side
lines of evolution that flourished for variable periods of time, such as Cyrtoceras, Gyroceras, Lituites, Phragmoceras, and Aseoceras. Nau tilus itself, which has persisted to the present day, was derived in early Ordovician times from one of these orthoeeraeones through a curved shell like Cyrtoceras. During the Silurian. De vonian. and Carboniferous periods the ortho eeraeones diminish in both size and number, and they disappear during the Twins. The deriva of Belemnites of the Alesozoie from some Paleozoic orthoceracones is considered by some authors to have taken place through the forma tion by the orthoeeras of a heavy deposit of lime upon the apical portion of its shell, this serving as a post to anchor the shell in an upright posi tion in the mud of the sea bottom. Consult: Enedeniann, "Professor Jockeys Theses on the :Mode of Existenee of Orthneeras and Other Ce phalopods," American Geologist, vol. xxxi. (Min neapolis, 1903) ; Von Zittel and Eastman, Text book of Paleontology, vol. i. (New York and Lon don, 1900). See CEPIIALOPODA ; NAUTILUS.