708 Mr. Stokes on Orthocerata. 



In those figured in the Geological Transactions, vol.i., 2nd series, PI. XXV., it will be seen that the 

 general form of the shell diminishes more rapidly in size, and that the siphuncle is perhaps nearer to 

 the side in fig. 3 than in fig. 1. In al! these specimens the septa are not preserved, but there is a 

 vacant space between each chamber, although the substance of the walls of the siphuncle are di- 

 stinctly seen. The substance of these septa must have been of considerable thickness, and on 

 breaking a specimen for the purpose of examination, I found that a layer of shell belonging to the 

 septa was visible on the cast of each of the chambers they had divided. From this it appears 

 that the septa are composed of several layers. 



S. Jctinoceras Richardsoni. (PI. LIX., fig. 2, 3.) 



In this the tube is small ; it is surrounded by numerous laminae or plates, filling up the siphun- 

 cle ; but it is difficult to determine their nature. 



The external shell is slightly conical in form. From Lake Winipeg, in yellowish-white lime- 

 stone much resembling that from Igloolik. 



4. Actinoceras Simmsii. (PI. LIX., fig. 4.) 



Shell large, conical, the upper chamber very deep*. The tube within the siphuncle smaller than 

 in A. Bigsbii. 



The number of the radii is not yet ascertained, but appears to be much less than in A. Bigsbii. 

 This is a very large species found by Mr. Frederick William Simms, in a lime-quarry at Castle 

 Espie, in the county of Down, in Ireland. It is two feet long, though imperfect, and contains the 

 siphuncle only in the lower portion of the specimen, being broken away in the other part. The 

 septa are composed of several laminae, and are of considerable thickness, showing a structure simi- 

 lar to that described by Mr. Miller in the chambered part of the Belemnite, but which he consi- 

 dered peculiar to that genus and not to exist in Orthocerataf. 



Of specimens belonging to other divisions of the Orthoceratidae, one from 

 Drummond Island, presented to me by Capt. Bayfield, R.N., is a most beau- 

 tiful and instructive example of the separation of a siliceous fossil from its 

 surrounding- rock by the long-continued action of weather. 



It exhibits an entirely new arrangement of the siphuncle, which could not 

 be shown in the external view of this part, but which is well laid open in this 

 fractured specimen. 



The structure thus seen has led to the explanation of peculiarities in the 

 siphuncle of several specimens which were previously unintelligible, and hence 

 to the establishment of another genus in the family, founded, as in the pre- 

 ceding case, upon the structure of this part of the shell. 



* I have noticed this circumstance here because it is very remarkable in this specimen, although 

 I believe that the upper is always deep in Orthocerata when perfect- In this instance it is how- 

 ever deeper than in any other I have seen. This figure is one-third of the natural size. 



t This specimen is remarkable for having the external surface of the shell covered to a consi- 

 derable thickness with a small species of coral, which must have continued for along time to grow 

 upon the shell, evidently after the death of the animal. This will be referred to in a subsequent 

 part of his communication, p. 710. 



