358 



An arctica-form with more prominent anterior end and 

 witii disappearing lunula; diagonal ribs as in the preceeding 

 form, sometimes however only traces of them. The maximum 

 length is 44 mm. 



A pholadis-form, oblong, with short, rounded anterior end,^ 

 elongated posterior end truncate behind, upper and lower margin 

 almost parallel, without or only with faint traces of diagonal 

 ribs. The maximum length is 50 mm. 



1 do not mean, however, that the arctica- and pholadis- 

 forms represent fixed subspecies, not to say species as some 

 authors mean. For we find transitions between them , both 

 with regard to the shape and the ribs. They may not either 

 be distinguished by the hinge, as we in typical pholadis -sc eci- 

 mens may find a rather distinct tooth. 



The East-Greenland specimens are exceedingly varying with 

 respect to the solidity of the shell. Some specimens — both 

 of the arctica- and the pholadis-fovm — have exceedingly 

 thick valves. The growth layers are very frequently markedly 

 segmented off at their border, so that it looks as if several 

 individuals had taken possession of the same shell, one within 

 the other. Other specimens have on the contrary utterly thin 

 valves, so fragile that they will break if not treated with care. 



Fossil it has been taken in the following localities by 

 the Danish expeditions: 



1891—92. Danmarks-Ö, uppermost in the shell-heap between the station- 

 buildings, ca. 20 feet above the sea-level. Some fragments. 

 1891 — 92. Danmarks-Ö, from a gravelly area across the isle. 4 valves. 

 1891 — 92. Morænepynt in Scoresby-Sound, till 120 feet above the sea-level. 

 Ca. 40 valves, essentially of the arctica-ïorm, partly thick-shelled. 

 1891 — 92. Morænepynt in Scoresby-Sound, from the uppermost terrace, in 

 cla.\, 120 feet above the sea-level. 1 valve, of the arctica-form. 

 1891 — 92. Rolige-Bræ near the Kobberpynt, inner Scoresby-Sound, ca. 75 feet 



above the sea-level. Some fragments. 



1891 — 92. Jameson-Land, in clay-banks, ca. 50 feet above the sea-level. 



Ca. 28 valves, mainly of an elongated arctica-îorm. 



1891 — 92. Jameson-Land, in sand-banks between Cape Stewart and Cape 



Hooker. Ca. 40 valves, of the arctica-form, partly thick-shelled. 



