L. 



of calcareous portion 



Whorls containing a calcareous 



layer 





1.4 



1 







1.9 



> 1 







2.1 



> 1 







2.2 



> 1 







4 



1 





68 N. ODHNER, NOETHERN AND ARCTIC INVERTEBRATES. VI. PROSOBRANCHIA. 2 SEMIPROBOSCIDIFERA. 



is evident tliat tlie shell attains its maximal size in tlie Jiehring Sea district and that it 

 varies most in the formation of the spire and in tlie structure of tlie cuticula. 



The spire contaiiis abou.t 2 whorls, is prominent or is sunk and wholly covered by 

 the epidermis (forma cryptospira) witli all transitions between tliese extremes. 



In comparison with V. velutina the present species has a smaller nnmber of whorls. 

 Tliis is due to the fact that the iniiermost ones of V. velutina are not developed in V. 

 plicatilis (just as is the case in V. lanigera). No sculpture is present on the einbryonic 

 whorls and the first portion of tlieni is always calcareous, wliile the shell otherwise con- 

 sists chiefly of a coriaceous substance. The calcareous portion embraces more or less of 

 the apex; its exteiit is evident from the following survey: 



Locality Length of shell 



Gullmarn 14.4 



Finmark . . • 8 



White Sea . . . • ... 9.5 



Bel Soiind 13..S 



Vega St. 1006 17.7 



The extension of the calcareous embryonal whorl is consequently, in relation to 

 the size of the shell, smaller in Gullmarn than in the arctic districts; but in some speci- 

 mens from the Swedish coast a calcareous covering occurs to some extent also in the 

 body-whorl. On the coast of Siberia sucli a layer, thougli extremely thin, occupies in 

 one specimen nearly all the inside of the aperture. 



The persistence of a calcareous layer in the embryonic whorls was observed even 

 by IMiDDENDORFP 1851, who based his species V. cryptospira chiefly on the strength of 

 this character. Judging from MiDDENDORFr's figm-es (Pl. 25, Figs. 8 — 10) and his de- 

 scriptions of the calcareous apex, concealed under a cuticular coating, that is considered 

 to form the specific character of V. cryptospira, this is entirely identical with V. plica- 

 tilis and V. flexilis, where the same conditions are always present, though in greater 

 or less perfection. 



The cuticula, it shoiild be noted, always covers the apex; where it is more sunk, liow- 

 ever, this covering is thicker. In its structure the cuticula shows a considerable variation 

 in the arctic districts, being smooth or appearing in longitudinal stripes or folds, or 

 formiiig spiral ridges (Siberian form). On the Scandinavian coasts no sculpture of the 

 cuticula appears; in some specimens, however, faint spiral ridges or rugse of the coria- 

 ceous layer may be scen. 



Tlte radula has in 3 examined specimens (Gullmarn, Bel Sound, Vega St. 1016) 

 a short and broad central cusp of the median tooth. The laterals have from about 5 to 

 7 strong denticles on each side. The uncini are smooth, slender; in the specimen from 

 Bel Sound the outer uncini are represented by very small rudiments. The radula of the 

 Vega specimen (St. 1016) is quite typical, its breadth is O.32 mm. 



