Mr. Stokes on Orthocerata. 709 



The interior of the siphuncle in these specimens is not continuous, but is 

 divided into separate portions, corresponding in number with the chambers. 

 The exterior shell of the siphuncle is extremely thin, and is generally broken 

 away where it has been exposed to the weather; and the interior portions 

 of the siphuncle are seen. A deep indentation in the middle of each of these 

 portions corresponds with each septum of the shell, so that of each of these 

 interior portions of the siphuncle one half is placed in one chamber and the 

 other half in the chamber below it. A longitudinal section of it exhibits a 

 very curious arrangement : the opening or interior passage of the siphuncle 

 is comparatively small, and the inner and outer lines of each portion shown 

 by the section are beautifully curved, as shown in PI. LX., fig. 1. The space 

 between the inner and outer walls of these portions is considerable, and there 

 is no trace of its having been filled by any substance. 



Induced by the information afforded by this specimen, I made sections of 

 others in which I have detected the same construction, and by this means 

 arrived at the knowledge of the external thin shell which originally covered 

 the siphuncle. The siphuncle thus formed, has some resemblance to a row of 

 beads, and hence the name of Ormoceras * has been chosen for a group or 

 genus to include them. 



As yet I can attempt to distinguish only three species, all of which are from 

 the same locality, Drummond Island in Lake Huron : 



1. Ormoceras Bayfieldi. (PI. XL., fig. 1.) 



Shell slightly conical. The laminae of the septa separate from each other on one side of the 

 shell so as to show a space between them. This will be afterwards noticed. 



2. Ormoceras Back'ii. (Geol. Trans., 2nd Series, vol. i., PI. XXX., fig. 1.) 



Siphuncle larger than the preceding. This species a good deal resembles the figure in Plate 

 VI. of Breyneus, Dissertatio de Polythalamiis. 



3. Ormoceras Wh'itei. (Geol. Trans., 2nd Series, vol. i., PI. XXX., fig. 2.) 



Named after M. White, Esq., of the Army Medical Staff, who was stationed on Drummond 

 Island, and collected this and many other specimens of these interesting fossils. 



In speaking of Actinoceras Richardsoni, from Lake Winipeg, 1 pointed 

 out the resemblance of its siphuncle to the Huronige, which I formerly con- 

 sidered to be corals, and described as such in Dr. Bigsby's paper. I was led 

 to that opinion by the radiating plates, for which I knew of no analogy except 

 in similarly radiating lamellae of the madreporic corals. I did not at that 

 time understand the peculiar structure of the centre of these corals, in which 



* From "Opfios monile, and Kepat cornu. 



4 y2 



