712 Mr. Stokes on Orthocerata. 



have found that it is of considerable length, and that it gradually diminishes 

 in thickness to the upper termination, where it is so thin that it probably was 

 never exposed. 



When speaking of Actinoceras Simmsii, I have mentioned that a beautiful 

 coral with small cells, resembling the Favosites, covers it, and which shows the 

 o-rowth of a long period ; there being many concentric layers accumulated on 

 the shell. This is the only instance I know of the occurrence of coral or shell 

 attached to an Orthoceras ; and the rarity of such cases may fairly be stated 

 as a further argument to prove that the Orthocerata were protected from the 

 adhesion of parasites. In this instance, indeed, there is evidence of the 

 growth of the coral being subsequent to the death of the shell ; for a portion 

 of the coral is covered with earthy deposit, over which other layers of the 

 coral have afterwards grown*, as shown in the section, PI. LIX., fig. 5. 



It has long been my wish to attempt a general revision of the Orthoceratidae, 

 with the view of arranging all the fossils of this family, and dividing them into 

 groups; but the subject is very extensive, the difficulty attending the investi- 

 gation is great, and my avocations allow me but little time to devote to it. I 

 have thought it better to limit myself to the preceding observations, and leave 

 them to the test of examination by other observers. I wish, however, to point 

 out another peculiarity in the structure of Orthocerata, which is to be found 

 in the septa of the chambers. The accompanying representation, PI. LX., 

 fig. 4, of one from Russia, is the best example 1 have met with. The laminae 

 of the septa in these cases appear to separate, and leave a considerable space 

 between them; on one side of the chamber the line of the upper lamina is 

 peculiarly curved, and seems to follow a regular law, as the form is similar in 

 each septum. 



I cannot at present say, if this peculiarity is confined to a particular division 

 of the Orthocerata. The Russian specimen may probably belong to the division 

 Ormoceras, which has the siphuncle divided into portions. This is not posi- 

 tive ; but the same peculiarity is seen in Ormoceras Bai/Jieldii and O. Whitei. 

 I have also observed a nearly similar arrangement in Liluolite. It must be 

 noticed, however, that as this separation of the laminae of the septa is only on 

 one side of the shell, consequently our knowledge of this peculiar struc- 

 ture is only to be obtained from the sections laid open by accidental fracture, 

 or by those made artificially, and mostly for other purposes than the ascertain- 

 ing of this point. 



* Since this paper was communicated to the Society, Mr. S. Stuchbury has commuHicated to 

 me another example in the collection of the Bristol Institution of similar coral surrounding an 

 Orthoceras. 



