on the Fosdls of the older Deposits in the Rhenish Provinces. 323 



of the species common to two of these systems is almost perfectly equal, whether 

 in reference to the first and second or to the second and third. 



VIIJ. a. Gasteropoda. — Like the Conchifera monomyaria and dimyaria, the Gaste- 

 ropod Mollusca were far from presenting during the Palseozoic such numerous genera 

 and species as during the following periods, and their feeble relative proportion is even 

 yet more striking. The Phyllidianee and the Calyptraciance have some representatives 

 in the three systems, but they do not possess any particular interest. Shells related 

 to the genus Melania are very plentiful in the mountain limestone of Belgium ; and 

 it is the same with the Natica, which are also very common in Yorkshire and in 

 the Rhenish provinces, and one species, the N. subcostata, nobis, is very abundant 

 in the Devonian beds of PafFrath. Some species are found in the Carboniferous 

 system of the north of Russia, and two in the province of Bolivia. Shells related 

 to Neritm have been pointed out in the Silurian beds of England, in the Devonian 

 system in England and at Elbersreuth, also in the mountain limestone of Belgium 

 and the British Islands. The Schizostomce, which begin the series of genera cha- 

 racterized by a fissure in the right lip, and at that time so well developed, are pecu- 

 liarly Devonian, but are still few in number. The 8. radiata, nob., is found in 

 Devonshire and the Eifel. 



We pass over some unimportant genera to come to the Euomphali, which were 

 very numerous during the three ancient periods. Seventeen are Silurian, and 

 are very plentiful in that system in England and Scandinavia. The E. rugosus. 

 Sow., E. funatus, id., and E. alatus, His., are particularly characteristic of it. 

 Among the seventeen of the middle system found in England, on the banks of the 

 Rhine and at Elbersreuth, the E. annulatus, Phil., alone is found both in Devon- 

 shire and in the Rhenish provinces. The species common to the two lower systems 

 have a wider geographical range ; the E. gualteriatus of Schlotheim, in particular, 

 is stated to occur at Christiania, in the islands of Gottland, Aland, and Dago, at 

 Revel, Paflfrath and Chimay. The Carboniferous system is still richer in species 

 than the preceding ; they are frequent in the mountain limestone of Yorkshire, 

 Belgium, the banks of the Rhine, and north and central Russia. The E. pen- 

 tangulatus and E. Catillus, Sow., are characteristic of this system throughout that 

 extent, and the latter, according to American authorities, is also found upon the 

 shores of Lake Erie and in the state of New York. The E. Lahadyei, nob., and 

 E. Serpula, de Kon., are found both in the Devonian and Carboniferous systems. 



Among the Cirri (a genus that we retain provisionally) there are also some 

 Devonian and Carboniferous species. The C. rotundatus, Sow. {Euomph. Dionysii, 

 auct.), is frequent in the beds of the upper system, and a conoidal variety is found 

 at Paffrath. 



The PleurotomaricB, still more numerous than the Euomphali, are very unequally 



