KUNGI.. SV. VET. AKADEMIENS HANDLINGAR. BAND 59. NIO 3. 53 



External morpliology (fig. 65). 



The mantle is thickened över the whole body, mostly towards the left 

 umbo. The tvvo siphonal openings together occupy almost half (or at least more 

 than a third of) the under margin. No siphonal fold is present but there is at least 

 an indication of the median part of one; the gills are fixed on the sides of the anal 

 siphon. The inspiratory opening is small, only V3 of the posterior adductor in length. 

 Both siphonal openings have their basal as well as their apical margins papillated 

 and pigmented, othervvise the mantle fold is smooth. The pedal slit is very small, 

 only Vs of the anterior adductor in length. 



Of the adductor muscles the anterior one is considerably the larger, the 

 posterior one being only about a /a of the former in length ; the former is also larger 

 than or equal to the distance between the upper ends of the adductors. The an- 

 terior adductor is broader at its upper part than the insertion length of the palps, 

 and the posterior one is narrower than the posterior gill. 



The labial palps are inserted at equal levels on both sides, at or beneath 

 (not above) the upper ends of the anterior adductor; the right ones are directed 

 dovvnwards, the left ones somewhat backwards ; the ends of both pairs reach down 

 to about V* or V« °f the anterior adductor. The lips are distinct, and the breadth 

 of the mouth is about twice that of the palps. 



The foot is lanceolate, narrow at the base, somewhat widened towards the 

 end; its breadth at the base is about l li of the length, and this is equivalent to 2 h 

 of the distance from its base to the ends of the labial palps. The foot projects 

 from a keel-shaped elevation of the frontal under part of the visceral hump. Behind 

 the foot a small knob-like swelling represents the pouch, and farther back, at a 

 distance from it equal to the length of the foot, a short triangulär lobe appears, 

 which may be free from or united to the gills. 



The gills almost completely cover the visceral mäss of the body. Those of 

 the specimen shown in the figure have their tips attached to the mantle on both 

 sides of the anal siphon but are separated from each other in the median line. The 

 margins of the reflected laminae are quite free (except in the posterior gill, which 

 is attached to the mantle belovv). In another specimen the gills, with their lower 

 ends, coalesce with each other and with the mantle, but the reflected laminae have 

 free margins. In two young specimens the gills were completely free. 



Interna] anatoiny. 



The intestinal canal. The oesophagus shows in a transverse section one 

 dorsal list and one ventral furrow as well as 5 furrows on each side. Just above 

 the outlet of the oesophagus into the stomach there debouches a short liver canal; 

 immediately beneath the oesophagus there appear two canals on the left and two 

 on the right, as well as one farther back on the right side; then the principal liver 



