8 TYPE AMMONITES— III Oct 



VJD 



and will be fairly concise, if special particularity is thought essential, 

 to describe, say, Cadoceras as platycone to cadicone, as short for com- 

 mencing as platycone and changing sooner or later to cadicone ; but 

 it would not be correct to describe Ldbyrinthoceras as cadicone to 

 sphaerocone, for the cadicone stage has dropped out owing to tachy- 

 genesis, just as in much specialized Liparoceratids the serpenticone stage 

 has done (Y.T.A. I, p. xiv). It would, however, be possible to describe 

 Labyrinthoceras as sphaerocone post-cadicone, and thus distinguish it 

 from Liparoceratids described in certain cases as serpenticone to sphaero- 

 cone, or in the abbreviated cases as sphaerocone post-serpenticone. It is 

 these abbreviated cases (saltative palingenesis, I, p. xiv) which may 

 lead astray and give trouble in too fine analysis, until ammonite genealogy 

 is much more fully illustrated. 



On the other hand, it may be pleaded that some cone-terms in 

 changeable species refer to no more than the outer whorl of the specimen 

 under consideration. Such, at any rate, is the case with those now 

 proposed : Ellipticone and Goniocone are only concerned with phenomena 

 belonging to the last whorl of full-grown specimens ; while Contracticone 

 is usually concerned with the difference between the last part of the last 

 whorl of a full-grown specimen and that of the preceding part, though 

 there are cases where the reduction began earlier even than the last whorl. 



Not all -contracticones, it may be remarked, are incipient scaphiti- 

 cones or scaphitoids : some are ; and, on the other hand, many scaphitoids 

 are not contracticones. 



Development 



With the above and other terms for shape, and with the terms for 

 size which have been proposed (III, 6), it is possible to make concisely 

 some remarks on the supposed habits and development of Cephalopods, 

 and especially of Ammonoids. 



The early Orthocones utilising the gas-effusion, which resulted from 

 temporary indigestion under the nervous apprehension of danger, found 

 that a cone thus made more buoyant was a help in rapid retreat from 

 foes. They brought the feature to perfection — a systematic gas-generator 

 and containing chambers. But retreat from foes was only incidental : 

 bottom-crawling in search of food was the normal occupation. As the 

 orthocones increased in size fear of foes was less insistent ; but the gas- 

 plant was still of service to lighten the load of the superincumbent conch. 

 Now progress through the water would cause a certain amount of drag 

 on the vertical conch, giving it a slant : there would then be tendency 

 to absorb test on the lower edge and to make more deposition on the 

 raised edge. Persistence of this tendency would give some curvature 

 to the conch, and would convert an orthocone into a cyrtocone, then 

 to a gyrocone. As both these stages would be inconvenient for bottom- 

 crawling or for darting away, there was good reason to hurry development 

 into the ophiocone : then the conch, coiled in contact, would be more 

 compact, and would be carried more easily, owing to the lowering of 

 the centre of gravity. Darting away would still be possible, and, when 

 ill circumstances coming again brought reduction in size and made such 

 a habit again more frequent, it may be surmised that the cut-water 

 peripheries of oxycones would be developed to facilitate rapid movement. 

 Sharpened peripheries mean strict, presumably forced, economy — the 

 attempt to attain the largest size, and to make the greatest show, with 

 the smallest outlay of material. 



It seems inadvisable to postulate quite the same habits for megalo- 



