6 SYNOPSIS OF THE CHARACTERS OF THE 



strata from the clialk to the lias inclusive, and the Goniatites, and gigantic Orthoceratites of the older rocks, 

 seeming to have been among the chief carnivorous animals of those periods. They have all nearly disappeared 

 from our seas. Nautilus and Spirula alone remaining to show us the nature of the animals which inhabited the 

 many chambered shells famihar to the geologist. The Siphonifera are in their internal anatomy most nearly 

 aUied to the Gasteropoda ; they differ remarkably from the Dibranchiata in ha\'ing four gills, and only one 

 heart, and most particularly in wanting the internal dorsal lamina, and being covered by an external polythala- 

 mous shell, the chambers of which are connected by a siphuncle differing in structure according to the family. 

 The subdivisions of this tribe are so numerous, and would lead us so far away from our subject, that it is better 

 not to notice them. 



Family OETHOCERATIDJE. 

 Genus Orthoceras. 



Gen. Ch. — Shell gradually tapering; smooth; siphuncle central. 



There appear to be several subgeneric types even of the genus as here restricted, the first of those is 

 Orthoceras proper (fig. 2), in which the form is straight conic, the section is circular or nearly so, the si- 

 phuncle central or nearly so, and the septa simple and placed at right angles to the long axis of the shell. 

 The econd is Loxoceras% JM'Coy (fig. 3), in which the section is oval, the septa waved and placed ob- 

 liquely with respect to the axis of the shell, and the siphuncle is excentric. The third is Trif/onoceras^, 

 M'Coy (fig. 4), in which the section is heart-shaped, the shell curved, with a concave back bounded on either 

 side by a distinct ridge or keel, surface smooth, and the siphuncle central. The fourth is Campyloceras", 

 M'Coy (fig. 5), in which the section is circular, the septa simple, and the surface smooth, but the shell 

 is curved and the siphuncle dorsal. Fifth, Cydoceras^, M'Coy (fig. 6), or those conical species marked with 

 prominent concentric rings, and having the surface frequently sculptured with transverse scaly lamina, and 

 often decussated; siphimcle dorsal. 



Fig. 3. 



Fig. 4. 



Fig 5. 



Fig. 6. 



Fig.7. 



Cb 



This family also contains the genera Poterioceras^, M'Coy (fig. 7), or the short fusiform orthoceratites, witii 

 contracted mouths, and Actinoceras, Stokes : the latter a doubtful genus. 



Orthoceras attenuatum. Flem. (Not of Soic.) 



Orthocera attenuata. Flem. An. Phil. 



Sp. CVi.— Slender, subcylindrical, very gradually tapering ; septa distant; surface very finely striated 

 transversely. 



This species is allied to the O.Steinhaueri, but is smaller and more slender, and the transverse striae 

 very much finer ; it is seldom more than the eighth of an inch in diameter. This is a more slender shell than 

 the O. elongato-cinctum of Captain Poiilock's Geological Report, which it otherwise very much resembles. 



Ao|o;, oblique, and xsp«;, a horn. 

 Kufi'x-vXai, incurved, and xs^a;, a born. 

 noTii^iot, a vase, and y.i^ui. 



'' Tpsr?, three ; yuyU, an angle ; and xspa;, a born. 

 ■^ KaxAo'5, a ring, and xsga;. 



