C o n c h i fe r a. 



3. Yoldia obtusa. M. Sars. 



(PI. 3 fig. 16—20). 



Yoldia abyssicola. M. Sars. ,,Clu"istiania Vid.-Selskabs Forhandlinger" for 1858 p. 8C. 



Tliis very distinct species, first discovered by M'. Conservator Koren in theHerlofjord 

 near Bergen at a depth of 300 fathoms, is also one of those peculiar animal forms which we 

 certainly oidy know from the superior limit for their occurrence; while in all probability it 

 exists very extensively in the greater depths of the Ocean. 



My father thought it expedient to change the previously used appellation abyssicola 

 and to substitute the above, in order to avoid confusion with the very different arctic variety 

 of the Yoldia pygma>a Miinster, to which the same name abyssicola had been given by Torell. ' 



The Slivll (fig. IG, 17) which in the largest specimens has a length of 13 millimotres, 

 is somewhat strongly compressed from the sides, very thin and fragile, and of a broad elip- 

 tical form: its length is nearly twice the height; and its thickness is considerably (nearly Vg) 

 less than the height. The sides are very inequal; as the umbones, which are small and slightly 

 prominent, are situated far in front, at the limit of the anterior third part of the length of 

 the shell. Anteriorly the shell is narrowly rounded, but posteriorly widened little by little, and 

 at last almost abruptly truncated. The ventral border forms in its anterior part, together 

 with tlie anterior border, a perfectly even curve; while further back it is more horizontal, 

 going over, with a very strong and sudden curve, into the posterior border, which is nearly 

 straight and perpendicular. The dorsal border is, in front of the umbones, somewhat in- 

 clined downward, going evenly over into the anterior edges ; but behind the umbones it 

 is quite straight and horizontal, forming with the posterior border a very strong nearly 

 angular curve. The shell is externally quite smooth, with extremely fine concentric lines 

 of growth, and covered with a strongly shining yellow green epidermis; internally it is of a 

 dull white color. 



The cartilage-groove is small and indistinctly defined, longitudinal, narrow, and not, 



' "Chr. Viil.-Selskabs Forhandlinger'- for 1868. p. 256. 



