Bela rugulata, Trosch. for sig at se, at den typiske brede, 

 coniske Tandform (Tab. IX, Fig. 1) forekommer langs Kysten 

 indtil Tromsø. I Finmarksfjordene afløses den, af Formerne, 

 afbildet i Fig. 3 — -4, der danner Overgangen til B. assimilis 

 G. 0. Sårs (Fig. 9, 10). Denne sidste er ikke funden sønden- 

 for Lofoten. Og saa kommer den rent arctiske Form spitz- 

 bergensis nob. (Fig. til med en temmelig afvigende Tand- 

 bygning. Alle disse Former horer de mindre Dyb til. 

 Dette var den horisontale Udbredning. I de større Dyb 

 i vore sydligere Fjorde træffe vi Formen bergensis nob, 

 (Tab. IX, Fig. 8) der afviger i sin Tandstruktur fra alle 

 de foregaaende. Af Arten B. scalaris Moll. har jeg lige- 

 ledes i de større Dyb fun det en Form, der har Tænder 

 væsentlig forskjellige fra den i de hoiere Vandlag (sammenlign. 

 Tab. IX. Fig. 17 den typiske scalaris med Fig. 21, 22 

 var. abyssicola). 



Den i Tab. IX, Fig. t— 5 af hildede Række Tænder til- 

 hører alle Bela rugulata forma typica; herfra til var. assimilis 

 Fig. 9 — 10 er kort Vei og ligesaa til bergensis Fig. 7 — 8. 

 Formen spitzbergensis staar eiendommelig isoleret. Tænderne 

 kan ogsaa variere hos det enkelte Individ, men. maa jeg 

 tilfoie, kun en Gang har jeg fundet dette i en paafaldende 

 Grad. De 3 Tænder Tab. IX, 9, 9 a og 9 b tilhører samme 

 Individ (B. assimilis) og, som en lille Dværg laa Fig. 9 a 

 imellem de ovrige Tænder; den var dog fuldt udviklet i Form. 

 Bela nobilis Moll. og scalaris Moll nærmer sig ogsaa saa stærkt 

 til rugulata hvad Tandstrukturen angaar, at de med denne 

 Karakter alene neppe fortjener hoierer Rang end de foran 

 nævnte Former, men her tror jeg ligesom .Sars og tiere 

 andre Forfattere^ at der er ganske paalidelige ydre Karak- 

 ter, der til enhver Tid vil ådskille dem. 



En Gruppe, som baade i ydre Former og Tandstruk- 

 tur synes at gaa lige nær over i hinanden som den netop 

 nævnte er B. Trerelyana, decussata. Kobelti, tenuicostata og 

 oralis. Tænderne byder her saagodtsom ingen paalidelige 

 Skillemærker, og der forekommer ogsaa imellem flere af 

 disse Arters Skalformer meget nære Overgange. 



Efter mine Erfaringer vil Belaformerne lade sig rangere 

 i følgende Grupper, som, hvad ydre Form og Piltændernes 

 Struktur angaar, viser nærmest Slægtskab. 



Rugulata Gruppen: 



Som fælles ydre Karakter kan fremhæves: Vindin- 

 gerne kjølede, de axiale Ribber rette (ikke sigmabøiede). Tæn- 

 derne varierer fra bred conisk til bred lansefonnig. 

 Arterne er: Bela rugulata Trosch. 

 „ scalaris Moll. 

 „ nobilis — 

 Cancellata Gruppen : 



Skallen har de axiale Ribber sigmabøiede; Tænderne 

 er sværdformige. 



Arterne er: Bela cancellata Moll. 

 cinerea — 



able that when we examine Bela rugulata, Trosch. we see 

 that the typical, broad, conical dental form (PL IX, fig. 1) 

 occurs along the coast as far North as Tromsø. The 

 forms illustrated in figs. 3 — 4 replace it. however, in the 

 Finmark fjords, and form the transition to B. assimilis 

 G. 0. Sars (figs. 9. 10). The latter is not found south of 

 the Lofoden Isles. And now, we have the purely arctic 

 form, spitzbergensis nob. (fig. (>) with a considerably diver- 

 gent dental structure. All of these forms pertain to the 

 shallow depths. This is their horizontal distribution. In the 

 great depths of our Southern fjords, we meet with the variety 

 bergensis nob. (PI. IX, fig. 8), which, in its dental structure, 

 varies from all the preceding ones. Of the species B. sca- 

 laris Moll. I have, also, in the greater depths, found a 

 variety having teeth materially different from that of the 

 shallow waters (compare the typical scalaris PI. IX, fig. 17 

 with var. abyssicola figs. 21, 22). 



The series of teeth illustrated in PL IX, figs. I — 5 

 pertains to Bela rugulata, forma typica; thence, to var. as- 

 similis, is only a short step, and likewise, also, to bergensis 

 (figs. 7. 8). The variety spitzbergensis stands peculiarly 

 isolated. The teeth can, also, vary in the individual animal 

 but, I must add that, only once have I found this to be the 

 case in a remarkable degree. The 3 teeth, (PL IX, figs. 9. 

 9 a and 9 b) pertain to one individual ( B. assimilis), and that 

 shown in tig. 9« lay, like a little dwarf, between the other 

 teeth; its form was, however, fully developed. Bela nobilis 

 Moll, and scalaris Moll, approach, also, so closely to 

 rugulata in regard to dental structure, that they, from this 

 characteristic only, scarcely merit higher rank than the 

 previously mentioned forms but. here, I believe, in accor- 

 dance with Sars and several other writers, that there 

 are quite reliable external characteristics, sufficient, at all 

 times, to distinguish them. 



A group which appears, both in external form and 

 dental structure, to pass as much over into one another 

 as that just mentioned is, B. Trerelyana,, decussata, Kobelti, 

 tenuicostata and oralis. Here, the teeth present almost no 

 reliable distinguishing features, and there occur, also, be- 

 tween the shells of several of these species very close 

 transitions. 



According to my experience, the Bela species may 

 be arranged in the following groups, which, both in regard 

 to external form and structure of the arrow-teeth, show the 

 closest relationship. 



The rugulata group: 



The whorls are angulated; the axial ribs straight, and 

 not sigmoid. The teeth vary in form from broad-conical, 

 to broad-lanceolate. 



The species are : Bela rugulata. Trosch. 

 „ scalaris Moll. 

 ,. )iobilis „ 



The cancellata group: 



The shell has its axial ribs curved sigmoidally. The 

 teeth are ensiform. 



The species are: Bela cancellata Moll. 

 „ cinerea 



