KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 50. N:0 5. 77 



front, at the sides of tlie mantle cavity. Posteriorly the mantle-border forms a short and 

 narrow groove, situated within the muscular bands; to this groove, the walls of whichare 

 clothed with a high secretory epitheKum, the shell is fixed by its thinnestpostero- median 

 edge. In some specimens of O. groenlandica there may be traced a bipartition of this 

 groove into two lateral centra. 



O. latissima differs in some respects considerably from the above-named species, 

 The mantle börder as well as the shell-muscle of both sides are not united posteriorly, 

 but terminate on the sides of the body. The posterior ends of the shell-muscles, which 

 are united with the marginal miiscle-band, are usiially produced in a projecting triangulär 

 lappet; at the upper and posterior side of these lappets the mantle forms an oval pad- 

 shaped dilatation, containing a winding groove of the same structure as the central one 

 just described in the preceding species. Thus in O. latissima a bijDartition of the origin- 

 ally single (shell-forming?) furrow has been carried oiit. 



The differing shape of the shells depends on these differences in the mantle-mar- 

 gin. In O. glacialis and O. groenlandica the shell is an oval plate with well -def ined 

 margins all round, except for a very short stretch in the central posterior part adhering to 

 the mantle-furrow. In O. latissima the shell is semi-oval, abruptly truncated posteriorly 

 in a straight line between the two posterior muscular lappets; towards its posterior mar- 

 gin it gradually attenuates and terminates without a definite limit. 



Mr. Francis Balch in 1910 described a new Labradorean species of Onchidiopsis, 

 which he named O. corys; it is characterized by its peculiar shell, which differs from that 

 of the other forms by having its posterior part reflected under the anterior one, The shell, 

 havnig the »consistency of a film of collodion» is similar to that of O. latissima, but 

 here a duplication is never found. 



According to Bergh, 1886 — 1887, the chief characteristics of the species O. gla- 

 cialis and O. groenlandica are the colour of the ospliradium (white or blackish) and the 

 structure of the inner uncini (denticulated in the outer margin, or smooth). In the pre- 

 ceding account some exceptions to these rules are pointed out. A close examination 

 has given rise to the conclusion that other characters of a constant specific import 

 are obtainable from the structure of the penis and the shape of the foot. 



The 'penis retains in O. latissima its simplest form as a cylindrical process of a slightly 

 S-formed curvation. The proximal or basal portion is often somewhat flattened to a 

 low crest-like expansion on its outer side; the distal part tapers regularly (cfrPl. 5, Fig. 4). 



In O. glacialis (Pl. 5, Fig. 5) and O. groenlandica (Fig. 6) the curvation of the penis 

 is more perfect, its end being reflected backwards (and terminating abruptly). Besides 

 this a differentiation in a basal, a median and an apical part is to be observed. The me- 

 dian zone has the sharpest curve. In O. glacialis it still retains a more primitive shape 

 owing to its comparatively slight dilatation in the lower margin; the apical part forms a 

 direct continuation of the median curve, not strongly distinguished from it, and is consider- 

 ably lengthened, so that its length much exceeds the breadth of the median part. In 

 O. groenlandica these relations are reversed, the median part being widely dilated to a 

 concave disc by a crest in its lower margin; from the upper and outer corner of it the 



