DE VERNEUIL AND d'aRCHIAC ON RUSSIAN PALAEOZOIC FOSSILS. 113 



almost the only representative of the class of Crustacea, and we can 

 scarcely even recognise a trace of other families. In Russia the 

 only exceptions are Limulus oculatus from the copper grits of Perm, 

 Eurypterus tetragonophthalmus from Podolia, and specimens of Cy- 

 therina, of which we have noticed several species in the Silurian, 

 Devonian and Permian systems. The whole interest with regard to 

 the group is therefore concentrated in the Trilobites, animals be- 

 longing, as is well known, to the most ancient fauna of the globe, 

 and presenting, like the genus Orthis, this remarkable fact in the 

 history of fossils, namely, that we are not able to discover any indi- 

 cation of their advance towards the state of perfect and full deve- 

 lopment. From the very earliest period they seem to have been in 

 full vigour, both in point of numbers and organization : they pre- 

 served for a time a great relative importance, they then became less 

 abundant, and ultimately they died out towards the termination of 

 that part of the palaeozoic period during which the coal-measures 

 were deposited. Owing no doubt to peculiar local conditions, this 

 diminution is more rapid in Russia than in countries further to the 

 west, and, in fact, after having attained in the lower part of the Si- 

 lurian system a development resembling that which they seem to 

 have enjoyed during the same period in Scandinavia and the British 

 Islands, the Trilobites diminish rapidly in the Russian Upper Silurian 

 rocks, almost entirely disappear in the Devonian system, and only 

 show themselves in rocks of the Carboniferous epoch under the pe- 

 culiar forms which have been included in the genus Phillipsia. 



Amongst the genera of Trilobites most widely spread in the neigh- 

 bourhood of St. Petersburg and on the shores of the Baltic, Asaphus 

 and Illmna are the most prominent, and of these, A. expansus and 

 /. crassicauda are the species most frequently met with, and these 

 are especially characteristic of the lower members of the Silurian 

 series. The vast thickness of tlie deposits in which they occur ex- 

 plains in some measure the wide extension of these species over the 

 globe, the latter species (/. crassicauda), to which we unite /. per- 

 ovalis, being found not only in Russia and Scandinavia, but in 

 England and Ireland, in Brittany, and even as far as Canada and 

 the United States of North America ; while the other species {A, ex^ 

 pansus), if it does not itself extend so far, still appears to be repre- 

 sented in Ireland and on the other side of the Atlantic by a nearly 

 allied form, A. platycephalus. Other species also, as A. Buchii 

 and A. iyrannus, are mentioned as occurring in the environs of St. 

 Petersburg, but these are less common, and are associated with 

 three new species discovered by the Duke of Leuchtcnberg. 



The genus Calymene has only afforded to our researches three 

 species, C. Fischeri, C hellatida and C. Odiui, all of them also cha- 

 racteristic of the contemporaneous beds of Scandinavia, and of these, 

 C. Fischeri is the most abundant at St. Petersburg. Besides these, 

 however, M. Eichwald and M. Pander have also mentioned five 

 others. Tiie genus IJro7dcs, of which the species oscillate between 

 the base of the Devonian and the uppermost beds of the Silurian 

 system, is only represented by one species, B.Jiabellifcr, found near 



VOL. II. — PART II. L 



