326 Viscount (I'Archiac and M. de Verneuil 



toides, d'Orb.), and B. apertus, Sow., common to the three systems in England, 

 Belgium, and on the banks of the Rhine, show their close mutual relations. In 

 North America this genus equally characterizes' the Palaeozoic rocks, not only in all 

 the northern states on the shores of the great lakes, but also of those which occupy 

 the centre of the great vale of the Mississippi, and on the west of that river even to 

 the foot of the Rocky Mountains in the Missouri, the Prairies and the Arkansas. 



VIII d. Cephalopoda. — If we judge of the number of animals of this class which 

 peopled the primary seas only by the remains of their solid parts which have been 

 preserved until our time^ we should readily regard them as nearly equal to the Bra- 

 chiopoda ; it is, however, probable that those seas furnished nourishment to great 

 numbers of naked Cephalopods which have left no evident trace of their existence. 



The Nautili are rare in the Silurian system : we have seen descriptions of only 

 four in Europe, of which one is stated to be found in the island of Gottland *, two 

 occur in Esthoniaf , and one in the Caradoc sandstone of Llandovery \. Only two 

 are Devonian, from the south-west of England ; but forty are Carboniferous, and 

 are distributed through the beds of this system in Ireland, England, Belgium, on the 

 banks of the Rhine, and they extend as far as Donetz and even to the Ural chain. 

 The iV. globatus, Sow., multicarinatus, id., cariniferus, id., tuberculatus, id., and sul- 

 catus, id., are those which are found at the most distant points. The species of 

 this genus found in the ancient beds are generally small, and some of them have 

 one or tw^o keels, a character which is no longer found in the secondary and more 

 recent periods, in which, moreover, the Nautili attain much greater dimensions. 



We will here regard the Orthoceratites as a great family, in which we shall arrange 

 as genera and subgenera those species which are more or less spiral at their summit, 

 more or less inflated toward the aperture, or those in which may be remarked a 

 siphon, whose singular and varied form indicates important modifications in the 

 organ which occupied it ; and we retain the name of Orthoceratite for those which 

 are straight and regularly conical. 



The Orthoceratites, properly so called, are extremely abundant in the deposits 

 anterior to the Carboniferous ; and the nearly equal distribution of the species in the 

 three systems is a remarkable circumstance, which we have yet had occasion to 

 notice only in* reference to the Terebratula. The forty-six Silurian species almost all 

 belong to England and Scandinavia ; a very few are found in the Upper Silurian 

 rocks of Esthonia§, Ingria, in the beds of Trenton Falls ||, and in those of Tennessee^. 



* Hisinger, Lethaea Suecica, tab. 7. fig. 1 ; Sowerby in MS. 



f Eichwald, Syst. Silur. Esthonie, p. 113. :J; Murchison's Silur. Syst., pi. xxii. fig. 7, p. 6't2. 



§ Eichwald sur le Systeine Silurien de I'Esthonie, 8vo, 1840, p. 99. 



II State Reports, New York, 1838, p. 115 ; 1840, p. 371. 



^ Troost's Memoir Org. Rem. Valley of the Mississippi, Trans. Geol. Soc. Pennsyl., vol. i. pt. 2. p. 248. 



