610 THE POPULAR SCIENCE MONTHLY. 



posed of simple laminae with smooth posterior edges. This last order 

 Agassiz subdivided into Acanthopterygian and Malacopterygian Cy- 

 cloids, 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 fos- 

 sil 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 uni- 

 formity of type as well as a uniformity in the different parts of the ani- 

 mals themselves. The Placoids and Ganoids were the only fishes then 

 inhabiting the seas ; but, as we approach the Jurassic period which be- 

 came 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 occu- 

 pies 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 Eng- 

 land 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'Echi- 

 noderms vivant et fossile," a most important contribution to modern 

 zoology. This work comprises five parts : the first and second, on the 

 Salenies and Scutellce, by Agassiz ; the third and fourth, on the Galerites 



