108 MR. G. C. CRICK ON THE MUSCULAR ATTACHMENT OF THE 



with tlie anterior border of tlie muscular scar. It may have been the line of attachment 

 of the j)osterior 2iortion of the body just prior to the formation of a new septum. 



Glypliioceras trimcatum, J. Phillij^s, sp. — An example of this species from the 

 Carboniferous Limestone of St. Doulagh's, co. Kildare, Ireland, which has been lent me 

 by Dr. A. H. Poord, T.G.S., also exhibits the anterior boundary of one of the muscular 

 scars A^ery distinctly (PI. 20. figs. 19). The specimen consists of about five-sixths of a 

 whorl, the whole constituting- part of the body-chamber, the base of which is fortunately 

 preserved. Its dimensions are: — diameter 75 mm.; height of outer whorl 36 mm. ; ditto 

 above jireceding Avhorl 20 mm. ; greatest thickness (which is at about the middle of 

 the lateral area) 27 mm. ; width of umbilicus 10 mm. On one side of the specimen the 

 anterior boundary of one of the muscular scars is preserved as an impressed line (on 

 the internal cast of the body-chamber), which arises from tlie suture of the shell at 

 about 4 mm. in advance of the last septum and curves gently outward and backward. 

 It can only be traced for a length of about 6 mm., when it is obscured by the test ; 

 at about one-half of this lencrth it is 45 mm. from tlie suture of the shell. 



Summary. 



As in the recent Nautilus, so in the Ammonoids, the shell was external and the 

 animal was attached to its shell by means of " shell-muscles " and an " annulus." 



In the Ammonites and such allied forms as BacuUtes, Samites, &c. the shell- 

 muscles Avere attached to the dorsal portion of the shell; they frequently either 

 approximated or met each other in the median line of this region ; when they did not 

 quite meet they Avere doubtless united by a more or less narrow band corresponding to 

 the dorsal portion of the annulus in the recent Nautilus. 



My observations support the conclusion expressed by Dr. Waagcn that the line 

 figured by Oppel on the body-chamber of Ammonites steraspis indicated the position 

 of the anterior boundary of the annulus and of the shell-muscles, the latter being 

 situated, as he supposed, upon the inner or umbilical portion of the lateral area of the 

 AA^iorl. It is, howcAer, not a little sti*ange that in the species figured by Oppel the 

 form of the muscular attachment differs somcAvhat considerably from that in the 

 majority of the Ammonites which I have examined. It would seem that another 

 intei'pretation is necessary for the figures Avhich have subsequently been given 

 purporting to be the remains of the muscular attachment. 



In the earlier stages of develoj)ment and in the general form of the shell as well as 

 in the aperture of certain species, affinities have been recognized betAA^een the Ammonoids 

 and the Dibrauchiatcs. Prom the foregoing it is clear that the Ammonoid animal 

 possessed a muscular attachment quite similar to that of the living Nautilus, the only 

 recent genus of the Tetrabranchiates. 



Indications of the muscular attachment of the Ammonoid animal, instead of being 

 rare, seem to be fairly common. There appears to be some ground for believing that 

 its form is in j^art due to the shape of the transverse section of the Avhorl and to the 



