618 



THE AMERICAN NATURALIST [Vol. XI J 



Spines. — These are of two kinds in gastropods. In a number 

 of phylogerontic platyceroids they consist of tubular prolongations 

 from the surface of the shell at irregular intervals, and appear to be 

 of little phyletic significance. The other, more important type 

 represents a periodic emargination in the lip of the shell (Figs. 3-4, 

 7-8). That the periodicity of this spine formation is connected 

 with the regular recurrence of the reproductive period, may be 

 assumed for types in which these spines appear late in the ontog- 

 eny; but not for those in which they appear early. Direct obser- 

 vations in this field seem to be wanting, however. The rate of 

 growth of the shell during the interval between such periods of 

 spine formation determines the number of spines upon a volution. 

 As has been shown for Fulgur (Studies II, p. 534) the number of 

 periods in progressive types decreases as the shell increases in size, 

 thus showing that the amount of growth between resting stages 

 (i. e., spine forming stages) increases at a rate faster than the rate 

 of increase in the size of the shell. This may of course also be 

 interpreted as a lengthening of the time interval between resting 

 stages, or possibly between the reproductive periods. In some 

 forms the decrease in the number of spine periods to the whorl is 

 a rapid one, as in Fvlgur eliceans, where it is 12, 9, and 6 respectively 

 for the 5th, 6th, and 7th whorl, whereas the decrease is 14, 13, 12, 

 or 13, 12, 11 for the same whorls in F. carica. In Murex (Rhino- 

 canthus) hrandaris, on the other hand, it is 6, 6, and 7 for the 5th, 6th 

 and 7th whorl respectively, thus showing a shortening of the growth 

 periods. In the majority of Murices, however, the number of 

 periods has been reduced to three for all whorls. In Ranella the 

 number of resting stages as expressed by the varices has been 

 reduced to two for each whorl. 



The simplest spine is that found in Fulgur. It has been quite 

 fully described \n a preceding number of these "Studies" (II) 

 and tlic only point tliat needs to be emphasized in this connection 

 is the u-nidiuil apjH urance of the spine in the ontogeny of the indi- 

 vi(hial. This i> not noticeable in FuUpir carica or other accelerated 

 types of Fulgur. as fullv discnss(Ml in the papers referre<l to, but is 

 well shown in th.- in.or.' primitive Miocene types le. g.. /•'. Irifouis, 

 fig. 7). Here the spines ap])ear on a laintly keeled or smooth shell, 

 and at first are in the form of the merest faint spinelet, scarcely 



