XVI UROCOPTIDyE. 



specialized genera, such as Pineria, seem to have passed 

 through this stage and regained a more primitive form, judg- 

 ing from the ancestry indicated by the anatomy of P. vie- 

 quensis. (4) The structure of the axis indicates that many 

 phyla have passed the acme of their specialization, and are 

 on the decline. Axial lamellae, etc., are protective in func- 

 tion, and evidently had their inception in the later whorls, 

 the soft parts retracting up beyond them, as in typical Holo- 

 spira (vol. xv, pi. 21, f. 31), or Anisospira (xv, pi. 11, f. 1). 

 By progressively earlier development they appear in the 

 young shell, until finally a lamella which at first occupied 

 the lower whorls only, extends throughout the shell, as in 

 typical Eucalodium. Now in some forms, such as Idiostemma 

 perlata (xv, p. 167), the most complex development of the 

 axis is located in the earlier whorls, the structure degenerat- 

 ing in the later. The species has passed its prime when the 

 axial structure was most elaborate. Similarly, in a large 

 proportion of the Gongylostomoid group the downward- 

 pointing spines of the axial lamella are obsolete in the median 

 and later whorls, but the earlier whorls retain them as minute 

 vestiges of formerly functional structures. (5) Sculpture of 

 the exterior is greatly modified in several phyla of the family, 

 the riblets being transformed into hollow ribs or bosses (see 

 xv, pi. 44, 45, 48, etc.), the acme of sculptural evolution in 

 Urocoptida. 



In the forms with a partially uncoiled last whorl, the latter 

 frequently retains a conspicuous "impressed zone," as Hyatt 

 has called the concave or flat parietal surface which in close- 

 wound spiral shells is impressed by the preceding whorl. 

 This appears as a concave surface in Eucalodium, etc., as a 

 sulcus or groove in many Urocoptis, such as U. (Callonia) 

 dautzenbergiana (vol. xv, pi. 48, fig. 6). 



The Urocoptidce, as a whole, seem therefore to have passed 

 their acme. The Urocoptina? especially, by the wealth of ex- 

 tremely and diversely specialized phyla, bring to mind the 

 specialized and retrogressive Ammonites of the Cretaceous, 

 and the outre Strombidoc of the later Mesozoic and early 

 Eocene, which have left in Rostellaria, Aporrhais, etc., only 



