634 



THE AMERICAN NATURALIST 



\you XLI 



out of the ontogeny altogether so that the angulation of the ribless 

 whorls and the appearance of true spines is almost simultaneous. 

 Yet the Eocene species indicate that the genus passed through a 

 normal series of round-whorled-ribbed, and angular-whorled-tuber- 

 culated stages before the spines appeared. As already noted, a 

 second row of spines appears in several lines of radiation in this 

 series. In the genus Rhinocanthus, typified by Murex (Rhino- 

 canthus) hrandaris, the principal spine likewise merges with the 

 tubercles which here are formed without the loss of the ribs. The 

 second spine has also been accelerated imtil it appears during 

 or shortly after the tubercled stage. In the more specialized 

 Murices, where the adult spines are compound, the early ones 

 have been pushed far back and are inseparable from the tuber- 

 cles or even from the ribs. It seems in fact that the spine-form- 

 ing stage has become superposed upon the rib-forming stage for the 

 ribs are characteristic of adult Murices of such relatively simple 

 types as M. hrandaris and of such highly complicated types as M. 



In tracing the phylogeny of spinous gastropods it must be borne 

 in mind that tubercles and spines have a different origin, and that 

 where they appear to merge into each other this is due to accelera- 

 tion. It is highly probable that the ancestral forms of such 

 types will be found to have these two characters separated, the 

 spines not being found in the earlier members of the phyletic 

 series as has been demonstrated to be the case in Fulgur. 



As has been shown above, the ontogeny of a great many widely 

 distinct types of gastropods is marked by a progressive increase 

 in the amount of embracing of the earlier by the later whorls. 

 This results in a change of angle of the spire from relatively acute 

 in young to often a rectangle or obtuse in the adult. In some 

 types (Conus) the change may be to 180 degrees, rarely more. 

 This same change is observable in the adults of the successive 

 members of the corresponding phyletic series. Thus the Eocene 

 species of Fusus are characterizefl by a slender spire throughout, 



cially in the young, than is shown in tiie modern species. A similar 



