ANIMAL TO ITS SHELL IN SOME POSSIL CEPHALOPODA. 73 



quite clear that it was furnished with shell-muscles and an annulus like the recent 

 Nautilus, and it is the object of the present communication to record the indications of 

 these structures in various Ammonoids. It is not proposed here to record every 

 Amnionoid in which these imj)ressions have been observed — this I hope to be able to 

 do subsequently — but to describe the form and position of these impressions so far 

 as I have been able to observe them in the different forms which the Ammonoids 

 assume, e.g. in BacnUtes, Hamitcs, Crloceras, Ancylooeras, Macroscapliites, Scaphites, 

 and Tiirr/lHes, the group of the Ammonites (ranging from very evolute to almost 

 entirely involute forms), as Avell as in Chjmenia and the group of the Goniatites. 



It may be well at the outset to refer to the indications of the shell-muscles and of 

 the annulus as they exist in the shell of tlie recent Nautilus, and for this purpose it 

 will probably suffice among the many figures which have been published of the m.uscular 

 attachment of the I'ecent Nautilus to call attention to the figures accompanying 

 Dr. Waagen's paper already alluded to (p. 71)*. I may, however, here remark that 

 in the recent Nautilus the shell-muscles are ear-shaped and situated upon each side of 

 the animal ; they are connected both on the dorsal and on the ventral side by a narrow 

 band — the annulus. The shell-muscles are not inserted into the shell-substance, but 

 are merely applied to the innei* surface of the test with the intervention of a thin layer 

 of conchiolin ; and all that is usually preserved in the interior of the shell to indicate 

 the form and position of the muscular attachment is a fine, generalh' raised line, corre- 

 sponding to the anterior boundary of the annulus and of each shell-muscle; it is 

 only rarely that there are any indications of the posterior boundary of these structures. 

 On an internal cast this raised line would be represented by an incised line, and since in 

 fossils the remains of the muscular attachment are preserved almost always on internal 

 casts, they therefore exist as incised lines. Such structures have been described and 

 figured in not a few fossil Nautiloids, including the genus Nantilns itself. In the fossil 

 forms any recoi'd of the form and jiosition of the muscular attachment would, when 

 present, usually be preserved iipon the surface of the natural internal cast of the body- 

 chamber, and hence raised lines on the inner surface of the original test would appear 

 on the internal cast as incised lines, and clce versa. 



In order to observe in the recent Nautilus the exact position of the muscular attach- 

 ment with relation to the edge of the last septum, an artificial cast of the shell ot 

 Ndiitilvs pompilius w^as made by filling a sagittal section of a recent shell with paraffin 

 wax, and then dissolving aw'ay the shelly matrix with hydrochloric acid. The anterior 

 boundary of the muscular attachment alone was indicated by a very finely incised line. 



To show the usual position of the muscular attachment in Ammonoids, and for the 

 better understanding of the less perfectly preserved examples, it is proposed first to 

 describe the impressions of the " muscular scars " in the specimen on which they have 

 been most clearly seen, and then the specimen in which the remains of the annulus 

 have been most clearly observed. The former is an example of Crloceras from the 



* See also the receiitlj'-published paper by L. E. Griffin, " Notes on the Anatomy of N(n(tilu.s pomjnlncs," Zoo]. 

 Bull. vol. i. no. 3, pp. 147-161 ; with bibliography. 



11* 



