16 GEOLOGICAL MEMOIES. 



Subgenus Huronia (Group 18). — Although our knowledge of 

 these fossils is somewhat limited, it is sufficient to indicate that the 

 sheUs of which they formed a part were " long-coned," and that in 

 the great development of their siphons they offer a remarkable con- 

 nexion, on the one hand with Endoceras, and on the other hand with 

 the Orthoceratites having a large nummuloid siphon, called Cochleati. 

 All the Euronice show a common and well-marked character, which 

 consists in each of their elements being composed of two very dis- 

 tinct parts, viz. : — 



1. Towards the base there exists a straight and cylindrical part 

 which adjusts itself in the aperture of the element placed imme- 

 diately below. This part, considered by itself, represents very well 

 an element of the siphon of an Endoceras, such as 0. duj^Ie.v, or its 

 analogous form 0. peregrinum. 



2. Higher up there is always a part more swollen (that is to say, 

 having a larger diameter), of which the rounded profile is comparable 

 to the ring of an Orthoceratite, adjusted upon the rectilinear and 

 lower part of the same element. This upper portion, isolated, re- 

 presents an element of a nummuloid siphon, like that of 0. cocJiJea- 

 tum, Schlot., or that of 0. nummidarium, Sow. 



Subgenus Endoceras, Hall. — In this subgenus are comprised all 

 those Orthoceratites with a large cylindrical siphon which charac- 

 terize the first phases of the second Silurian fauna upon the two 

 Continents, and which have been designated in the north of Europe 

 Yaginati, and in America Endoceras. The species of this subgenus 

 seemed for some time to possess a second characteristic in the mar- 

 ginal position of their large siphon. But the discoveries by Prof. J. 

 Hall of E. irroteiforme, with a large siphon, placed at a distance from 

 the side of the shell, and by ^I. Eichwald of a Eussian species, 

 E. Tiasta, with a large central siphon, besides the discovery of a repre- 

 sentative in Bohemia (0. iiovator), prove the inconstancy of this organ 

 in these fossils. 



Endoceras (Group 19). — Characterized by the great dimensions 

 and cylindrical form of the siphon. This group is provisional, as 

 the external appearance of the shell is not known. 0. peregrinum, 

 Barr., 0. novator, Barr., 0. insidare, Barr., 0. duplex, Wahl. 



Endoceras (Group 20). — The common character of this group 

 consists in the very pronounced rings upon the external surface. 

 Upon the European forms these rings are accompanied by striae 

 which foUow the same direction. The three types belonging to the 

 great northern zone are 0. vaginatum, Schlot., 0. trochlearis. His., 

 Endoceras anmdatum. Hall, while 0. Marcoui, Barr., has been disco- 

 vered by !M. Jules Marcou in the calcareous lenticles of the schists 

 of Phillipsburgh, Canada, upon an horizon which appears to corre- 

 spond to the origin of the second fauna. 



Subgenus Gonioceras, Hall. — Up to the present time this group 

 is represented by the single species figured and described by Prof. J. 

 Hall in 1847, under the name of Gonioceras anceps. The two prin- 

 cipal characters which distinguish this subgenus are (1) the very 

 flattened conformation of the transverse section, of which the axes 



