UROCOPTID^. XV 



lets in the direction of growth-lines on the pillar. These 

 riblets sometimes break into granules, as in some species of 

 Ccelocentrum, sometimes become wide-spaced nodes, or in 

 combination with spiral sculpture are transformed into spines 

 (Ccelocentrum astrophorea, Gongylostoma) or hooks (Idio- 

 stemma) ; or the vertical sculpture may be obsolete except 

 on the crests of the spiral lamellae, as in the crenate lamellae 

 of Amphicosmia (xvi, pi. 3). 



The column may be either solid, as in all Urocoptina, or 

 hollow, as in most genera of Eucalodiince. When the axis 

 is hollow, oblique white lines may be usually seen in its bluish 

 or gray substance, caused by the local thickenings of the latter 

 by striae or riblets inside the cavity or tube of the axis. 



SIGNIFICANT CHARACTERISTICS AND EVOLUTION OF THE 



UROCOPTID^:. 



PHYLOGERONTIC CHARACTERS. Many genera of Urocoptidae 

 are in the stage called by Hyatt phylogerontic. They are 

 in the old age of the race, and there is every reason to be- 

 lieve that many phyla in the family will not outlast the 

 present geological period. The stigmata of decadence are 

 seen on many sides. (1) The specialization of the radula 

 in all Urocoptince indicates an adaptation to special condi- 

 tions, probably too extreme to survive any material change 

 in environment. (2) The fusiform shape of the shell indi- 

 cates decreasing growth-power. This is well illustrated by 

 Holospira, in which the greatest diameter of the shell is often 

 above the middle, the later whorls successively reduced, some- 

 times overhung by those above. Just as overhanging whorls 

 mark failing nutrition of the individual, so decreasing calibre 

 in the species indicates ebbing vitality of the race. Cf. 

 Tetrentodon and Brachypodella, vol. xv, pi. 62, etc. (3) The 

 straightened, more or less uncoiled last whorl has the same 

 meaning. It has long been recognized that laxity of coil, 

 the tendency of the last whorl to project in a rounded neck, 

 is a feature of senility in the individual gastropod or cepha- 

 lopod, and of old age in the race. This tendency is almost 

 universal in the TJrocoptida, but some otherwise highly 



