70 N. ODHNER, NORTHERN AND ARCTIC INVERTEBRATES. VI. PROSOBRANCHIA. 2 SEMIPROBOSCIDIFERA. 



half of the periphery of the aperture. Its central or posteri or part, which is fixed on 

 fco the columella, is narrow, the two side portions widen forwards and terminate, bent out- 

 wardly, at the sides of the niantle cavity. The columellar part of the niuscle forms at 

 the right side of the body an ascending fold, before it passes över into the right lateral 

 part; this fokl corresponds to the inner (and upper) angle of the aperture. This descrip- 

 tion is apphcable to V. velutina. In V. undata the ascending angular fold of the miiscle 

 is lower. The two species have as a common character that the lateral muscle bands 

 are of about equal length and parallel (as seen from above), thus usually not converging 

 forwards. In V. lanigera the columellar part is only slightly expanded but no angular 

 fold is observable, the posterior (connecting) band is narrow and the lateral bands of about 

 equal length converge. In V. plicatilis, Siberian form, the columellar part is not par- 

 ticularly widened and on the right side is rounded, passing över directly, without angu- 

 lar fold, into the lateral band; the medio-posterior band is obvious and the two late- 

 ral bands are of about equal length and convergent. In V. plicatilis from Gullmarn the 

 upper part of the columellar portion is somewhat widened, the lateral parts converge 

 forwards, and the left muscle is longer than the right one, which terminates with a de- 

 flected end. Thus we find an expansion of 1) the före part of the animal in V. velutina 

 und V. undata (muscular bands parallel), and 2) the median part in V. lanigera and V. 

 plicatilis, this latter accompanied by a tendency to form a uniform semi-circular band 

 by leveling its angle on the right side. 



' The mantle. The colour of the mantle is partly of the same liglit yellowish tint 

 as the whole animal (V. plicatilis, V. lanigera), partly darker, from light grayish, uni- 

 form or dotted, to dark violet, interspersed with lighter areas (V. velutina, V. undata). 

 In V. velutina a darker mantle is combined with a darker brown shell, a lighter with a 

 light yellowish one. 



The mantle is attached to the shell margin by a narrow börder encircling the 

 whole animal close beneath the muscle-band. Below this börder the mantle is highly 

 swollen and wrinkled by transverse and longitudinal folds; this secondary mantle börder 

 is more or less reflected upon the shell, especially in V. plicatilis. Among the transverse 

 folds one stronger one is discernible, occupying the whole breadth of the thickened bör- 

 der and broader than the wrinkles; it will be found at the left side of the body close 

 to the foremost end of the gill, somewhat in front of the shell-muscle end (V. velutina 

 and undata). In this we recognize an inspiratory fold. On the opposite right side of the 

 body a similar larger furrow will usually be found close behind the anterior end of the 

 right muscle; this expiratory fold is, however, often obsolete. Behind it there is a larger 

 longitudinal fold or cavity. The position of these folds, wliich have been overlooked 

 by observers and which are of systematical import for the genus, varies somewhat in 

 the different species: in V. velutina the left (inspiratory) fold lies in front of the 

 left muscle- end, the right (expiratory fold) likewise before the right one; in V. undata 

 and insculpta the left fold lies in front of, the right one behind the före end of the 

 respective muscles; in V. lanigera and V. plicatilis botli folds are placed behind the mus- 

 cle-ends (in specimens from Gullmarn they are indistinct). 



As to the primary mantle-edge, this is of somewhat different extension in the 



