THE 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Akt. XII. — The Molluscan Archetype considered as a Yeliger- 

 UJce Form^ with discussions of certain points in Molluscan 

 Morphology 'j"^ by A. E. Yerkill. 



A NUMBER of years ago Professor R. Lancaster published 

 diagrammatic figures and a description of what he called a 

 " schematic mollusc " or an " archemollusc." He distinctly 

 stated, however, that he did not claim that it was an archetype 

 in the sense of having been the actual original form from which 

 all later mollusks have been developed. f His archetype has, how- 

 ever, been assumed by many later writers to represent the 

 actual primitive or ancestral mollusk, from which all modern 

 forms may have been derived. 



It is my present purpose to give some reasons for consider- 

 ing such an idea as unwarranted by our actual knowledge, and 

 to explain what I believe to have been the actual archemol- 

 lusks, or primitive forms of Mollusca. 



Observation and experience in many other cases show that 

 we cannot expect to find an archetypical or primitive form of 

 a great and diversified group among the highly specialized 

 adult forms now living. We should look for such primitive 



* Abstract of a paper read before the National A.cad. of Science, Washington, 

 D. C, April, 1896. 



f Encyclop. Britannica, ed, ix, vol. xvi, p. 635, 1883. His statement is as fol- 

 lows : " Such a schematic mollusc is not to be regarded £v ^"'"JV^i iii the 

 sense which has been attributed to that word, nor as an id^ present to a creat- 

 ing mind, nor even as an epitome of developmental laws. Were knowledge 

 sufficient, we should wish to make this schematic mollusc the representation of 

 the actual molluscan ancestor from which the various living forms have sprung. 

 To definitely claim for our schematic form any such significance in the present 

 state of knowledge would be premature, but it may be taken as more or less 

 coinciding with what we are justified, under present conditions, in picturing to 

 ourselves as the original mollusc or arche-mollusc." 



Am. Jour. Sci.— Fourth Series, Vol. II, No. 8. — August, 1896. 

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