BARRINDE SILURIAN CEPHALOPODA OF BOHajHIA. 



11 



variation as the fundamental basis of a particular generic type. In 

 certain species the modification simply indicates the age of the indi- 

 viduals ; for in Cyrtoceras the transverse section is circular in the 

 young specimen, while it becomes elliptical or oval or subtriangular 

 in adult age. Genera, therefore, such as Campyloceras, M'Coy, Tn- 

 gonoceras, ]M'Coy {Orthocera i^aradoxicum, Sowerby), Temnocheilus, 

 M-Coy, and Trematodiscus (subgenus), which have been founded on 

 differences of the form of the transverse section, are merged in their 

 proper places among the old-established genera. 



The aperture of the shell of the Xautilidae is sometimes simple, or 

 like the transverse section, and sometimes complex, or contracted 

 into two orifices. This difference seems to furnish a character as 

 easy as it is certain for the distinction of the genera of this family. 

 The genera which correspond in the two series of the Table (p. U) 

 are alike in every other respect, and are only separated by the con- 

 trasting form of their aperture. The contrast seems to the author 

 to indicate a different conformation of the important organs which 

 surround the head of the Cephalopod, and even of the head itself, 

 which appears to have been sometimes free and sometimes imprisoned 

 in the shell. In consequence of these conclusions the author has 

 combined with the genus Cyrtoceras the forms named Oncoceras and 

 the genus Streiotoceras of Mr, BiUings. 



The singular appearances produced in the siphon of certain Xau- 

 tilidae, and especially in the siphon of Ortlioceras, by organic de- 

 posit, has led to the formation of several genera, such as Actinoceras, 

 Onnoceras, Huronia, Emlocerns, ttc. All these appearances can be 

 traced to the normal and habitual forms of the elements of the siphon ; 

 consequently these generic denominations have been carried by the 

 author to the synonomy of the genus Orthoceras. 



The type Trochoceras is extejided to contain those Xautilidae 

 the shell of which presents a double cui've. Among the extreme 

 forms some are bent in contiguous volutions, and are strongly tur- 

 reted. Others, on the contrary, are simply bent, like Cyrtoceras,y^\th.~ 

 out completing a turn of the spire, and their second coil (?. e. their 

 curve in helix) is not manifest, except by a defect of symmetry. 

 Thus the genus Trochoceras represents among the Nautilida? three 

 types, distinguished by three names among the Ammonitidae, namely: — 



Tarrillfes, Lam., vnth contiguous volutions. 



Heteroceras, D'Orb., with contiguous volutions, except the last, which 



is extended in a straight line. 

 HelicoceraSf D'Orb., with separate volutions. 



This extension of the genus Trochoceras may appear carried too 

 far, if it is compared with that of the genera with a single coil, 

 among which are maintained : — 



Cyrtoceras, a type simply bent, with less than a turn of the spii'e. 

 Gyroceras, a type coiled, vdth disjointed volutions. 

 Nautilus, a type coiled, with contiguous volutions. 



The author admits that, from a purely theoretical point of view, 



