XXXV. On chondrosteus, an extinct genus of the sturionidæ, found in the lias formation at Lyme Regis

Abstract

Head covered with bony plates. Body short, deep, adpressed, devoid of dermal plates. Scapula and coracoid united. Dorsal fin advanced. Tail heterocerque. Fin-rays for­mula: P. 50; V. 50; D. 75; A. 30; C. 75. Endoskeleton partly ossified, partly carti­laginous. The recognition of a member of the Sturgeon family in the fossil state is due to Professor Agassiz, whose quick perception detected in some fragmentary specimens, found in the lias shales at Lyme Regis, structural peculiarities which convinced him that this family was not unrepresented in the secondary strata of Great Britain. The specimens first discovered were portions of the caudal fin, and it was from the examination of these that he established the genus Chondrosteus , and determined its affinities to the Sturgeons. This result is noticed at page 280 of the 2nd part of the 2nd volume of the ‘Poissons Fossiles,’ but no description is there given; nor has the subject been since alluded to by the Professor, the engagements undertaken in the United States having put a stop to the preparation of the supplementary continuation of his great work on which he was engaged previous to his departure. Most of the specimens subsequently discovered appertain also to the caudal region. The massive proportions of the caudal fin, the strength of the fin-rays, and the thickness and hardness of the fulcral scales on the upper lobe, account for the preservation of these parts in more perfect condition than others less qualified to resist the destructive agencies to which they have been exposed. Recent discoveries have disclosed a chain of evidence connecting these specimens with others, some of which were formerly assigned to the Sauroid and Ccelacanthoid famihes. The first link was afforded by a specimen which, in addition to the lower lobe of the caudal fin, contained the anal fin; subsequently a specimen came to light having the ventral fins superadded to the anal and caudal fins; and lastly, a fine examp e in hard has, showing the posterior part of the cranium, both pectoral fins, both ventral fins, the dorsal fin and the anal fin (Plate LXVII.). The snout and the tail are deficient. The study of this specimen, and more especiahy of the scapulocoracoid arc an is appendages, led to the identification of many other specimens of the an P which were not before supposed to have belonged to this genus. The facts brought to light by these discoveries, and the more extended observations based thereupon, fully corroborate the conclusion arrived at by Professor Agassiz, that this genus, although differing generically from the Sturgeons of the present epoch, belonged, nevertheless, to the same family. These facts I now proceed to particularize in detail, commencing with a general description of the most perfect specimen, and proceeding step by step through the anatomical characters displayed in the several specimens I have examined, so far as their state of preservation will allow.