Introduction. 



The present work forms a contiiiuatioii of the INIonograph entitled Prosobrancliia, 

 1. Diotocardia, which was published in 1912. It comprises the groiip Semiproboscidifera, 

 of BouviER (1887), and has been prepared on the same principles as the earlier parts of 

 this series, deaUng with MoUusca, that were issued in 1907 and 1912. The chief purpose 

 of this work being to give an account of the variation of the species, it has been the aii- 

 thor's task in the first place to subject the shell to examination according to the same 

 method as was described in the part that appeared in 1912. In order to give a com- 

 parison of .the sizes of the shells collected in the different districts, I have, in some cases, 

 adduced statements as to the number of whorls, my reason for so doing being the suppo- 

 sition that an equal number of whorls in two specimens represents the same stage of 

 development in them, and that these only are really commensurable. By this means, one 

 is enabled to compare the rapidity of the increase taking place in different regions and to 

 state where it attains its highest degree, where it goes forward normally and where it 

 proceeds more slowly; in the last case the species becomes dwarfed. Thus, I have found 

 that some arctic shells, above all Naiica clausa, which attains a considerable size in high 

 arctic districts, becomes dwarfed in southern areas, but that the progress of increase 

 may be intermediary in råte in intervening regions. 



The measurements of the shells are taken somewhat differently in the various 

 genera owing to the varying shape of the shell; thus »length», »breadth» and »height» 

 as here used have not always the same signification as respects the specimens. In ^^a- 

 tica the shell is viewed with the columella vertically ascending in the spire; the height is 

 the distance from the apex to the lowest part of the base (parallel to the columellar axis), 

 the breadth is (perpendicularly to the height) the distance from aperture margin dia- 

 metrically to the outer body wall. In Velutiria and similar forms the shell is thought 

 of as resting upon the aperture as its base; the length is the greatest distance from spire 

 region to aperture margin (thus falling obliqiiely to the columellar axis), the breadth 

 is measured perpendicularly to the length in the basal plane; the height is the distance from 

 the superior point of the body wall perpendicularly to the basal plane. The measurements 

 taken are thus not homologous in all cases, but I have disregarded this inconsistency 



