Page:Popular Science Monthly Volume 4.djvu/628

610 of simple laming with smooth posterior edges. This last order Agassiz subdivided into Acanthopterygian and Malacopterygian Cycloids, or fishes having two dorsal fins, one spiny and the other soft, and those having one soft dorsal fin. Agassiz found that the study of fossil fishes exhibits a remarkable parallelism between the development of the individual and that of the class in geologic time. During part of the embryonic life of fishes, and even in some adult forms, the dorsal cord exists as a simple gelatinous cylinder, surrounded by a fibrous sheath, in which, after a time, there is found a cartilaginous and then an osseous deposit, which goes to form the vertebrae, the ossification taking place first in the apophyses. This embryonic character Agassiz found to be peculiar to the fossil fishes of the earlier geologic ages. There is no trace of a vertebra, but the apophyses, usually ossified, rest directly on the spinal cord.

Regarding the permanence of type, the author found the species of one formation specifically distinct from those of another, and, while it is impossible to say that the species pass from one into another, as they appear and disappear suddenly without direct connection with their predecessors, yet, as a whole, they present a continual progress of development, from the lowest to the highest, and demonstrate most palpably the existence of an ever-present directive intelligence.

Up to the end of the Jura epoch there exists among fishes a uniformity of type as well as a uniformity in the different parts of the animals themselves. The Placoids and Ganoids were the only fishes then inhabiting the seas; but, as we approach the Jurassic period which became preeminently the age of reptiles, we find a remarkable abundance of Sauroids, which, in their osteological character, the organization of their soft parts, and their dermal integuments, approach so nearly the reptile Saurians. At the end of the Jura period we find the Ganoids and Placoids giving way to the Ctenoids and Cycloids, which at present constitute the majority of our fishes. In the chalk-group, two-thirds of the species belong to extinct genera; in the inferior tertiary, one-third. In the Norfolk clay and Molasse formations the genera, for the most part, approach those of the tropical seas of the present day; and in the Geodian clay of Greenland there is found a species identical with one now living. In addition to the description of the species, which occupies the bulk of the work, a chapter is devoted to a critical review of the fishes of Monte Bolca, and another to those of collections in England and Scotland.

Agassiz next turned his attention to the study of Mollusca and Echinoderms, and in 1836 published a prodromus of the Echinoderms, and in 1837 a treatise on the fossil Echinoderms of Switzerland. In 1839 he began a more elaborate work, entitled "Monographies d'Échinoderms vivant et fossile," a most important contribution to modern zoology. This work comprises five parts: the first and second, on the Salenies and Scutellæ, by Agassiz; the third and fourth, on the Galerites