1894. CEPHALOPOD BEGINNINGS. 435 



Piloceras and Actinoceras. As Clarke justly says of Piloceras, " were it 

 possessed of such a solid apical sipho as is Nanno, that would be the 

 part most readily preserved, as in this case." We have, however, 

 more definite reason for believing that no such solid protoconch was 

 present in Piloceras and Actinoceras. Figure 5, II., and figure 6 show 

 at the end of the conch a wide aperture with a raised rounded edge. 

 It is the opinion of Foord, with whom I heartily concur, that the 

 siphuncle passed through this opening into the protoconch (g, pp. 159 

 and 166). The non-retention of the protoconch in a fossil state 

 therefore seems due to the fact that it was composed of thin con- 

 chiolin, just as we have already inferred the protoconch of the 

 ancestral Nautiloidea to have been ; while the raised rim is remark- 

 ably like the rim of the cicatrix in a recent Nautilus. In these cases, 

 not only was the protoconch more fragile, but the secondary infilling 

 of the siphuncle tube did not extend into the protoconch as it did in 

 Nanno and as it is here considered to have done in Orthoceras elegans 

 and 0. politum. 



It appears a legitimate conclusion from the facts here quoted, 

 that, however far back we trace the Cephalopoda, we still find two 

 divisions : the one characterised by a relatively solid protoconch, 

 often of relatively large size, and often preserved in a fossil state; and 

 the other either possessing no protoconch, or one that was formed of 

 material too delicate to be preserved, but characterised instead by an 

 aperture with raised rim or by its degenerate representative, the 

 cicatrix. To the former division are to be referred all the undoubted 

 Ammonoidea and Coleoidea, while to the latter division are to be 

 referred all undoubted Nautiloidea. We are therefore not entitled to 

 say that the Ammonoidea were derived from the Nautiloidea, 

 although we may not doubt that all three orders sprung from a 

 common ancestral stock first evolved in far pre-Cambrian times. 

 Of course it follows from this that the St. Cassian species referred to 

 Orthoceras by Klipstein, as well as the specimen shown in Fig. 2, which 

 Clarke refers to the same genus, cannot belong to the Nautiloidea 

 at all, but are ancestral forms of the Ammonoidea and possibly of 

 the Coleoidea. It is indeed probable that many other species hitherto 

 referred to the loosely defined genus Orthoceras, or to some other 

 genus of presumed Nautiloid affinities, will eventually have to find 

 another resting-place. This is a question for those with more special 

 knowledge than I possess. 



Although in this article some adverse criticism has been bestowed 

 on my Transatlantic friends, yet the facts that they have brought to 

 our notice have a very real value. They have shown that the prin- 

 ciples that have hitherto guided us have not been erroneous, and they 

 have extended the basis of our argument by the width of a Hemi- 

 sphere. I feel therefore some confidence in once more making a 

 suggestion (3) originally placed before the Geologists' Association in 

 March, 1888, but which, since it was not considered worthy of pubh- 



2 F 2 



