806 THE EQUIANGULAR SPIRAL [ch. 



or axial portions of the whorls tend to form a soHd axis or 

 "columella"; and to this is attached the columellar muscle which 

 on the one hand withdraws the animal within its shell, and on the 

 other hand provides the controlhng force or trammel, by which (in 

 the gastropod) the growing shell is kept in its spiral course. This 

 muscle is apt to leave a winding groove upon the columella (Fig. 373) ; 

 now and then the muscle is spht into strands or bundles, and then 

 it leaves parallel grooves with ridges or pleats between, and the 

 number of these folds or pleats may vary with the species, as in the 

 Volutes, or even with race or locahty. Thus, among the curiosities 

 of conchology, the chank-shells on the Trincomah coast have four 

 columellar folds or ridges; but all those from Tranquebar, just north 

 of Adam's Bridge, have only three (Fig. 392)*. 



The various forms of straight or spiral shells among the Cephalo- 

 pods, which we have seen to be capable of complete definition by 

 the help of elementary mathematics, have received a very com- 

 phcated descriptive nomenclature from the palaeontologists. For 

 instance, the straight cones are spoken of as orthoceracones or 

 bactriticories, the loosely coiled forms as gyroceracones or mimo- 

 ceracones, the more closely coiled shells, in which one whorl overlaps 

 the other, as nautilicones or ammoniticones, and so forth. In such 

 a series of forms the palaeontologist sees undoubted and unquestioned 

 evidence of ancestral descent. For instance We read in Zittel's 

 Palaeontologyf : " The bactriticone obviously represents the primitive 

 or primary radical of the Ammonoidea, and the mimoceracone the 

 next or secondary radical of this order " ; while precisely the opposite 

 conclusion was drawn by Owen, who supposed that the straight 

 chambered shells of such fossil Cephalopods as Orthoceras had been 

 produced by the gradual unwinding of a coiled nautiloid shell {. 

 The mathematical study of the forms of shells lends no support to these 



* Cf. R. Winckworth, Proc. Malacol Soc. xxiii, p. 345, 1939. 



t English edition, 1900, p. 537. The chapter is revised by Professor Alpheus 

 Hyatt, to whom the nomenclature is largely due. For a more copious terminology, 

 see Hyatt, Phytogeny of an Acquired Characteristic, 1894, p. 422 seq. Cf. also 

 L. F. Spath, The evolution of the Cephalopoda, Biol. Reviews, viii, pp. 418-462, 

 1933 . 



X Th is latter conclusion is adopted by Willey, Zoological Results, 1902, p. 747. 

 Cf. also Graham Kerr, on Spirula: Dana Reports, No. 8, Copenhagen, 1931. 



