Mr. Stokes on Orthocerata. 1l(yi 



as in the species of Thessalon Island; and this is the first instance of this plan 

 of organization I have met with in an Irish or English specimen. 



As all these specimens agree in the peculiarity of the siphuncle possessing 

 an interior tube, and as this tube had doubtless an important function, I am 

 disposed to include them all in one genus ; and that I may not multiply names 

 without necessity, I will adopt the generic name of Actinoceras, given to the 

 species from Thessalon Island by Bronn in the Letheea Geognostica, where 

 he copies the figure from the Geological Society's Transactions, which figure 

 appears to be his only authority for establishing the genus*. 



It is true that the radii surrounding the tube within the siphon are not at 

 all detected in some of the species, and that the radiating lamellae of others 

 present a very different appearance from the separate radii of the species 

 from Thessalon Island, but the important peculiarity of a tube within the si- 

 phon induces me to include them in one genus, as a subdivision of the family 

 of the Orthocerata. More accurate knowledge may be acquired and the ar- 

 rangement now proposed may be improved, but in the mean time it will, I 

 think, be advantageous to adopt this character as a basis for making one group 

 of these interesting fossils. The name is not characteristic of all the species 

 thus brought together, but at present I had rather retain one that has been 

 published than introduce a new one. 



The character of the genus Actinoceras is a large siphuncle, much dilated 

 in each chamber and contracted at the parts where the septa are attached to 

 it. Within the siphuncle a continuous tube, which appears to have been ca- 

 pable of expansion or contraction, and (sometimes at least) furnished with radii 

 in verticillations which connect the tube within the walls of the siphuncle. 



The specimens 1 am yet acquainted with, appear to resolve themselves into 

 the following species : — 



1. Actinoceras Lyonii. (PI. LIX., fig. 1.) 

 In this species a small tube is seen within the siphuncle, but no radii have been traced from it. 

 The exterior has not been seen, and the specimens are not sufficiently perfect to lead to a decided 

 opinion on this point. The shell is conical, tapering gradually. Found at Igloolik and Ooglit, in 

 a yellowish-white limestone. 



2. Actinoceras Bigsbii. (Geol. Trans., vol. i. N.S., PI. XXV.) 

 The tube larger than in the preceding, and the radii in distinct verticillations. Found at Thes- 

 salon Island, Lake Huron. It may be doubted whether all the specimens belong to one species. 



* However desirable it may be to give names to fossils in order to facilitate references, I thmk 

 the attempt to establish and nair.e genera from no other materials than figures published by other 

 persons, is likely to lead to much evil. In the present instance Herr Bronn concludes his generic 

 description with the words, that it requires further confirmation. 



VOL. V. — SECOND SERIES. 4 Y 



