Ploui'otomidii 



Gen. Bela Leach. 



Ved Bestemmelsen af Arterne inden denne Slægt maa 

 man uundgaaelig Iægge til Grundlag Prof. G. 0. Sars's 

 Værk „Mollusca regionis arcticæ Norvegiæ"; thi der er intet 

 tidligere Arbeide, hvori disse nærstaaende Arter saa noir 

 er gransket og hvori er givet saa omhyggelige Tegninger. 

 der gjør en Identificeren mulig. 



Prof. Sårs har lagt en afgjørende Vægt for Artsbe- 

 stemmelsen i Piltændernes Struktur og omend jeg maa ind- 

 rømme, at enhver Karakter, der er konstant, den være 

 tåget fra Dyrets indre eller ydre, er berettiget som Arts- 

 mærke, saa er det paa den anden Side lidet tilfredsstillende 

 for Oonchologen kun at have med indre anatomiske Karak- 

 terer at gjøre og man kan ikke være tjent med at lade et 

 dodt. men velbevaret Exemplar henligge ubestemt, fordi 

 Tandbygningen ikke har været undersøgt. Ensidig indre 

 Karakterer er ikke praktiske. Tandstrukturens Stabilitet 

 maa man ogsaa kunne overse før man kan indrømme en 

 Arts Berettigelse, hvor der er Brist paa tilfredsstillende ydre 

 Skillemærke. 



Jeg har gjort mig til Opgave at skaffe saa god Over- 

 sigt som muligt over Tændernes Variationer og til den 

 Hensigt havt et meget stort Materiale af Slægten Bela 

 til min Raadighed. Ikke alene har jeg disponeret over de 

 rige Fund fra Expeditionen, Bergens Museum og min pri- 

 vate Samling, men min Ven Conservator Schneider har 

 været saa genereux at stille Tromsø Museums meget rig- 

 holdige Materiale af Belaarter til min Disposition. 



Inden Familien Buccinidæ har jeg paavist. at Tæn- 

 derne er en lidet paalidelig Karakter og at de er under- 

 kastet store Variationer inden Arterne. 



Hos Pleurotomidæ synes det ikke at være saa slemt, 

 men Karakteren er langtfra constant og der er al Grund 

 at være varsom med Artsdannelser, der har Piltændernes 

 Struktur som væsentlig Grundlag. Uden at ville fastslaa det 

 sum fuldt ud constateret, skal jeg her dog nævne, at det synes 

 sum ora de geografiske og de bathymetriske Forholde, 

 eller med andre Ord de klimatiske Forholde og et paa Grund 

 heraf forandret Levesæt har nogen Indflydelse paa Tainl- 

 strukturen. Ganske paafaldende er det, naar man tager 



Den norske tfordhavsexpedition. Herman Friele: Mollusca. II. 



Pleurotomid.M 



Gen. Bela Leach. 



In distinguishing the species included in this genus. 

 we must, of necessity, take as the basis. Prof. G. 0. Sar's 

 work — Mollusca regionis arcticæ Norwegiæ — because there 

 is no earlier work in which these closely related species 

 are so precisely investigated, and in which such careful 

 illustrations are given to enable their indentification to 

 become possible. 



In arriving at a determination of the species. Prof. 

 Sars has laid a vital stress upon the structure ol' the arrow- 

 -teeth. and whilst I must admit that every characteristic 

 which is constant — whether pertaining to the interior or 

 exterior of the animal - - is entitled to consideration as a 

 specific feature, it is. yet. on the other hand, little satis- 

 factory to the malacologist to have to do with the internal 

 anatomical characteristics only, and we can not be satisfied 

 with permitting a dead, but well-preserved, specimen to 

 remain undetermined because the dental structure has not 

 been examined. Restricted internal characteristics are not 

 practical. We must. also, have a complete survey of the 

 stability of the dental structure, before we can admit the 

 title of a species where there is a deficiency in satisfac- 

 tory external distinguishing features. 



1 have made it my study to obtain the widest pos- 

 sible survey over the variations of the teeth and. for this 

 purpose, have had a very abundant material of the genus 

 Bela at ray disposal. Not only have I had the rich col- 

 lection from the Expedition. Bergens Museum, and my pri- 

 vate collection, but my friend Mr. Schneider. Curator of the 

 Tromsø Museum, has been so kind as to place Tromso 

 Museum's very rich collection of Bela species at my disposal. 



In the family of Buccinidæ, I have shown that the 

 teeth are a little reliable characteristic, and that they 

 experience great variations in the different species. 



In the Pleurotomidæ this does not appear to lie so 

 grave, but the dental characteristic is tar from constant and 

 there is every reason to lie careful in the formation of species 

 having the structure of the arrow-teeth as their material 

 basis. Without affirming that it is fully determined, I will, 

 here, however, state, that it appears as if the geographical 

 and the bathymetrical conditions or. in other words, the 

 climatic conditions, and a consequent changed habitus, have 

 some influence mi the dental structure. It is quite remark- 



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