Clarke — The Naples Fauna. 



55 



Tendency to Form Pilae in Gerontic Stages. Old shells, or at least those 

 of a size passing normal adult dimensions, when 

 served, show in occasional instances the gradual 

 assumption of obscure concentric ridges, visible only 

 over the dorsolateral slopes and having the aspect 

 of Low fascicles of fine concentric lines. This ten- 

 dency is never carried to a notable degree of develop- 

 ment, but it is a character of the final growth 

 stages only. 



The precise significance of this superinduced or- 

 nament is not clear. While it may be of interest to 

 institute a comparison between it and the duplica- 

 tion or f asciculation of the secondary lamella) in the 

 neanic stage, it is unwise to attempt the interpreta- 

 tion of one in the light of the other or to regard 

 these pilae as anything else than accompaniments of 

 senility. The gradual development of pilae or their 

 allomorphic expression as nodes is seldom seen 

 among the goniatites, though we may cite a well defined instance in the 

 Agoniatites ewpansus, Vanux. var. nodiferus, Hall (See Palaeontology of New 

 York, vol. vii, Suppl. pi. cxxvii, fig. 7). 



In the early Styliola fauna, where, as we have already had occasion 

 to observe, the goniatites seem to be in a large degree aggressively accelerated, 

 we find a small form with neanic narrow umbilicus and also neanic whorl 

 section, but with a pilate or subnodate final whorl in which it was perfectly 

 evident that the pilae have originated in the earlier ornament of the surface. 

 This is the species, Mantic. nodifer, Clarke. 



The hyponomic sinus of the venter is indicated in the late neanic substage 

 by a broad backward sweep of the lamella?. During ephebic growth this sinus 

 w as intensified by gradually narrowing and deeper curvature. In full-growth 

 the now fine, concentric surface-lines are very strongly bent backward ami 

 make a deep festoon on the venter, their lateral curvature being so long and 

 the lines on the curve in so close apposition that final growth stages present 

 the appearance of bearing revolving lines at the margins of this hyponomic 

 sinus. These features are seldom well retained as the lines become constantly 

 more obscure with age, but they are shown in some of the accompanying 

 illustrations. 



the exterior is fully pre- 



Figure 23. The protoconch viewed from 

 near its distal extremity, attached to an 

 external cast of the early whorls. This 

 specimen shows the smooth surface of the 

 nepionic shell, the simple ornament of the 

 next succeeding whorl and the double 

 series of varices following thereon, x 18. 



