CLASS V CEPHALOPODA 593 



In some Paleozoic Nautiloids with large siphuncles (Endoceras, Actinoceras), 

 the apical end of the siphuncle is solid and dilated to form the nepionic bulb 

 (Hyatt), and this sometimes practically fills the camerae, and besides being 

 very large in a number of succeeding chambers. The endosiphuncle expands 

 near the apex in these genera, and forms a good-sized conical perforation or 

 cicatrix, which is obviously open at its termination (Actinoceras, Nanno). 



Closely coiled shells have the apical part bent so as to enclose a vacant 

 space (the umbilical perforation) in the centre of the whorls (Fig. 1119). 

 This is present in all the Nautiloidea having this mode of growth, although in 

 some genera it is very minute. The Nautiloid shell is invariably cone-shaped, 

 but this may be straight or curved, or coiled in open or closed spirals, but in 

 rare instances it is even screw-like, or similar to a Gastropod shell. Along 

 with perfectly smooth shells, or those marked only with fine growth-lines, 

 which in some rare cases may retain traces of their original coloration, there 

 are others with external transverse ridges, keels, rows of tubercles or laminae ; 

 but this ornamentation is of a simple kind, and never attains the degree of 

 complexity observed among the more highly ornamented forms of Ammonoids. 



Classification. — Great importance has always been attributed to the external 

 configuration and curvature of the shell in distinguishing genera, and the 

 principal groups usually named Orthoceras, Cyrtoceras, Gyroceras, Nautilus, etc., 

 have been founded upon such characters. Barrande emphasised in addition 

 the shape of the aperture, direction of the funnels, and structure of the 

 siphuncle, but considered these subordinate in most cases to the general 

 form, and the majority of writers have followed his example. Hyatt, 

 however, regarded the general form and involution of the shell as relatively 

 minor characters, and depended upon coincidence of structure, outlines of the 

 aperture, and especially resemblances in developmental stages, as surer guides 

 to the affinities of the species and characteristics of the genera. 



Terminology. — For sake of convenience, it is preferable always to speak of 

 the embryonal shell as the protoconch, and the later or epembryonic stages of 

 the shell as the conch, the term " shell " being really applicable to the entire 

 external skeleton inclusive of the protoconch. The history of the individual 

 and its shell can be divided into the following stages and substages : The 

 embryo or protoconch ; the nepionic stage or infancy, represented by the apical 

 part of the conch ; neanic stage or adolescent part of the more mature cone ; 

 ephebic or adult stage of the same ; and gerontic or senile stage with which it 

 terminates in a complete example. 



All of these stages differ materially from each other as a rule, and it is 

 often convenient to divide them into substages, connoted by the prefixes ana-, 

 meta-, and para-. Thus the nepionic can be separated into ananepionic, meta- 

 nepionic and paranepionic, and it is often essential to treat the neanic and 

 gerontic stages in the same manner.^ 



The many different forms of Nautiloid shells may be grouped into a few 

 leading types, as follows : An orthocone is the young of the straight as well as 

 of many of the coiled forms. In this, although straight, the bands of growth 

 are broader on the venter than on the dorsiim, and there is no hyponomic 

 sinvis. A cyrtocone is the similar stage which replaces or, as is oftener the 

 case, succeeds this and is curved. Both of these may have crests in the bands 



1 For a more extended discussion of terminology that can 1 >e advantageously used in descriptions 

 of shells of this class see Hijatt, A., Phylogeuy of an Acquired Cliarat:teristic, 1894, p. 422 ct seq. 

 VOL. I 2 Q 



