96 



LAMELLIBRANCHIATA. 



It seems as if the periostracum were thickest and most frayed in arctic specimens, as this 

 characteristic is seen not only in the specimens from East Greenland (and in part those from West 

 Greenland [and Iceland]), but also in (my) specimens from Spitzbergen and the Kara Sea 1 ). 



Sculpture. The shell may be, apart from the fine lines of growth, quite smooth or more or 

 less distinctly folded, at the umbones only or more or less far down on the shell. The folds sometimes 

 appear as fine and dense, sometimes coarser ribs with wider intervals, and these ribs may sometimes 

 in the adult specimens reach at least halfway down on the shell. 



Form. At Iceland the form is relatively high and more or less convex, as will be seen 

 from the following measurements: 



Height Breadth 



Locality Length Height Breadth 



Length Length 



Vestmannaeyjar 44 mm . 38 n,ra . 86.4 °/ 17 mra . 38.6 % 



Reykjavik 43 - 36.2 - 84.2 - 17.75 - 4*-3 - 



42 - 34 - 81 - 15.5 - 36.9 - 



38 - 33-5 - 88.2 - 17 - 44.7 - 



Faxafjordr ^ - 32.25 - 87.2 - 14.25 - 38.5 - 



36.5 - 32.5 - 89 - 16 - 43.8 - 



3475" 29 - 83.5 - 16.5 - 47.6 - 



Seydisfjordr 44.25 - 34.25 - 77.4 - 20.25 - 45.8 - 



At West Greenland similar forms occur, but here also — though apparently not very fre- 

 quently along the more southern (Danish) part of the coast — we find a greatly compressed form (var. 

 placenta Morch 2 ) = Astarte Richardson i Reeved), of which I may give the following measurements 

 as example: 



Height Breadth 



Locality Length Height Breadth 



Length Length 



West Greenland 39 n,m . 31 mm . 79.5 °/ o 12.25 """• 3 J -4 % 



At East Greenland we find more or iess compressed, often strikingly elongated forms 4) 



as will be seen from the following measurements: 



Height Breadth 



Locality Length Height Breadth 



Length Length 



'i" 



Turner .Sound 44 mm . 35 ™ m . 79.5 °/ o 18 n,m . 40.9 % 



40 - 3° - 75 " I4-25- 35- 6 " 



Tasiusak 32.5 - 27.2 - 83.7 - 13 - 40 - 



29 - 24.5 - 84.5 - 10.5 - 36.2 - 



Hekla Havn 29 - 21.5 - 74.1 - 9.5 - 32.8 - 



Forsblads -Fjord 27 - 20 - 74.1 - 8.25 - 30.6 - 



') C. G. J oh. Petersen's statement regarding A.borealis in Danish waters seems to me of interest in this connection: 

 "the epidermis is here (i.e. in specimens from the Belts and Baltic) much more frayed than in the Kattegat specimens'' (Det 

 vidensk. Udbytte af Kanonhaaden Hauch's Togter, 1893, p. 75). Compare also in this connection A. Krogh: "The tension of 

 carbonic acid affecting the structure and preservation of molluscan shells'; Medd. 0111 Gronland, XXVI, 1904, p. 387. 

 1 orch: Catal. des Moll, du Spitzberg. Mem. Soc. Malacol. Belgique, T. IV, 1869, p. 22. 

 Reeve in Belcher: The last of the Arctic Voyages, II, 1855, p. 397, PI. 33, fig. 7. 

 1) For such specimens Posselt rl. c) has set up a special variety, sericea, on account of the elongated form of the 

 shell, the thick epidermis and its silk-like sheen; in my opinion it cannot be kept separate from the variety placenta Morch. 



