Xo. 490] ORTHOGENETIC VARIATION 



)23 



portion of the shell, it represents the primitive condition and the 

 straightened part represents the later condition. Thus the prog- 

 ress of ontogenetic development is from close-coiled to non-coiled, 

 from which we are justified in deducing that the ancestor of the 

 type with the enrolled apex was a closely coiled type, and that the 

 loss of power to coil, shown in the adult, is a sign of old age of the 

 branch which that individual represents. That the earliest types 

 were non-coiling shells cannot be doubted: there is every reason 

 for believing that they were patelloid in form, though modern 

 Patella is a phylogerontic type, which in its adult characters has 

 returned to the condition of its forefathers. This is clearly shown 

 by the presence of the coiled protoconch which at once stamps this 

 form as derived from a coiled ancestor. All deductions then 

 based on the anatomy of the soft parts of Patella, which leave this 

 fact out of consideration are necessarily faulty, since it is hardly 

 conceivable that in all its characters this animal has either remained 

 primitive, while the shell passed through a tremendous series of 

 metamorphoses, or that the soft parts have likewise degenerated to 

 such an extent that they have reached in all characters the condi- 

 tion of the primitive ancestor of the gastropods. 



Another relationship that may be deduced from the structure 

 of the protoconch of many hi-li'ly ornamented types of shells, is 

 that tliey were derived from an ancestor with simple ril)s on rounded 

 whorls. Thus tlie ancestors of Fusus, certain Mnrices, Latirus, 

 Tudicula and other types were fusoid sltelis with simple ribs on 

 smooth rounded whorls, if the structure of tlie protoconch of these 

 types can be regarded as an indication of ancestral conditions. 

 For in these types the last portion of the protoconch is ribbed with 

 delicate vertical ribs l)ut witliout spirals. ^Yhile the community 

 of descent of tlie pMicra citc<l from some early Mcsozoic ancestor 

 is probable, it docs not follow tliat all ribbed j)rotoconchs indicate 

 a common ancestry. For when it is recalled that ribs appear 

 independently in tiie most divergent tyj>es of shells, it need not 

 surprise us to find that this primitive ribbe<l character has been 

 pushed back into the protoconch stage in a lunnber of different lines 

 of descent. 



There are, of cotn-sc, not wantiii-- those wlm d.-ny that the char- 

 acters of the protoconch can be regarded as indicative of phyletic 



