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THE AMERICAN NATURALIST 



[Vol. XLI 



and open to observation, whereas in the coiled cephalopod shells 

 the outer whorls must be removed before a satisfactor}' view of the 

 inner ones can be obtained. Despite this fact, the study of shell 

 ontogeny has been mostly confined to the cephalopods, especially 

 to the highly complex and often richly ornamented ammonite shells; 

 and this can hardly be wondered at, when we consider their 

 number, and^the high degree of diversity found in this group of 



Orthogenesis in the Ontogenetic Development of 



The individual development of gastropod shells always follows 

 one of a small number of lines of variation. Leaving aside color 

 markings, which have been somewhat fully discussed by the Count- 

 ess von Linden,^ we may devote this discussion to the changes in 

 form and ornamentation. At the outset, however, we must note 

 that in every perfect shell there are two parts to consider; namely, 

 the protoconch and the conch. These may be continuous with 

 each other, in which case the features of the protoconch merge into 

 those of the conch; or they may be discontinuous, when the proto- 

 conch features end abruptly and the conch features begin as 

 abruptly. (See Studies of Gastropoda, I, fig 5.) 



The form of the gastropod shell is manifold, but the types are 

 few. Primitive types always begin with rounded whorls free from 

 all ornamentation. The coiling of the whorls at first is in a suffi- 

 ciently loose manner to produce a hollow axis, opening below in 

 an umbilicus. Even in types in which the anterior end of the 

 adult is produced into a canal — as in Fulgur, Buccinum, Fascio- 

 laria, etc. — the earliest protoconch stage has been found to show 

 the umbilicated round-whorlcd condition (Stiulies of Gastropoda 

 I, fig. 3). This con(Hti()n may be acc()m{)anie<l by various modi- 

 fications in tlie anioiint of (>inbracing of the .succeecHiig whorls. 

 In all cases a nu.dcnite ainou.it of cmln-acing by the succeeding 

 wMs seems to be the simplest condition, although it nuist be borne 



1 Die Entwickclung der .Skulptiir uiul der Zeichmint; l>ei den Cehaiisc- 

 echnecken de^ MeoiX's. Z( U.^clir. j. wi^nuisch. Zoolo,jir, LXI, p. 201. 



