616 



THE AMERICAN NATURALIST \yoi.. XLI 



when withdrawn it must be folded into minute wrinkles to be- 

 come accommodated to the smaller space. That these spirals 

 are generally \isibie only as emarginations on the margin of the 

 lip, indicates that only the outer portion of the mantle is affected 

 by this excess of growth. If other parts of the mantle are affected, 

 lirae result on the inside of the outer lip and plications on the 

 columellar lip. 



The order of appearance of the spirals is a very regular and prog- 

 ressive one. In simple or primitive shells a single spiral makes 

 its appearance at the ambitus, followed in most cases successively 

 by spirals above and below. A partial exception to this rule is 

 found in types in which a certain number of stages have been 

 dropped out between the protoconch and conch and hence the 

 conch does not begin at the beginning (Studies I, fig. 5) but at a 

 stage normally belonging later in the ontogeny. Thus in Fusus, 

 Latirus, Semifusus and certain ^Nlurices the conch begins abruptly 

 with rounded ribs and three or more spirals. Such dropping out 

 of early stages is however entirely in conformity with the general 

 progress of development and is itself progressive, or orthogenetic. 

 It belongs however in the category of phyletic orthogenesis. 



At whatever stage of development the shell l)egins, that develop- 

 ment is thereafter progressive. I'hus in a large number of forms, 

 if the shell begins with three spirals, the fourth, fifth and later 

 spirals appear progressively above tlie up|)er, and below the lower 

 one. If the whorls become asynnnetrieal, with continued growth, 

 as is the case in the majority of shells wliere the lower (anterior) 

 portion is either drawn out into a spindle and canal, or is Hattened 

 and depressed, the development of the spirals will })e unecjually 

 distributed. Thus in shells with a spindle, three or four s])irals 

 may appear successively below, while only on(> app(\irs above the 



In tlii. rr.,,!.., tlu-v'n.MMnM,. MM-on.h.n ^nin.U f. uhi.'h indeed 

 tl.ry ni.MV br . loM.lv iiiiir.l. 'I'Ih v \^u^^v^vv .|,ii, klv iva.-h llu' sizC 

 an.l oth. r cli.-iia. nT^ of il,r tu.. liiM ^piral-< uitli uln.-li tlicy appear 



calated spirals with the habitat of the se<-()n(lary spirals appear 

 between them at a later period. 



