REPORT ON THE MOLLUSC A. 11 



In the gill of Mcdletia (PI. I. fig. 8) we no longer see the binary character of 

 typical Pelecypoda (except Tellinacea, Luciuacea, and Anatinacea). The branchial axis 

 (containing the two afferent and efferent vessels, and fixed to the visceral sac and to the 

 dorsal side of the mantle), as in Nucula and Yoldia, bears lamellae (or large liranchial 

 filaments) on each side, hut in the same longitudinal plane. Here, as also in Nucula,^ 

 the lamellae or filaments of one side make an obtuse angle with the plate formed by the 

 lamellee of the other. In the allied forms. Area, Peetunculus, Limopsis (PL II. fig. 2), 

 the two plates formed by the filaments are parallel, and almost touching each other ; 

 that is, they exhibit the typical binary arrangement of the gills of Pelecypoda. 



On the other hand, the gill of Malletia has all its lamellaj or filaments in the same 

 plane, right and left of the axis, and it presents an aspect similar to that of the gill of 

 Cephalopoda and Gastropoda {Fissurella and Haliotis). The gill of Malletia, therefore, 

 well represents a primitive gill, from which all the gill-forms of the Pelecypoda may 1)e 

 derived by successive specialisation. 



The great development of the labial palps, and the presence of appendages to these 

 organs, in Malletia and all Nuculidse, is in direct correlation to the structure of the gills. 

 These do not form the large plates, with ciliated surfaces, which play so large a part 

 in nutrition, by conveying the food towards the mouth in the currents which they 

 produce. The gills being exclusively respiratory organs, and having nothing to do 

 with nutrition as they have in other Pelecypoda, the latter function is performed by the 

 palps alone, and it is therefore quite natural that they should be so greatly developed. 



What can be seen, without dissection, of the rest of the structure agrees with that 

 existing in Yoldia. 



2. Yoldia isonota, Yow'Ma.xt&u^. Kerguelen Island ; 95-110 fathoms. 



I have used this species to compare with Malletia jfcdlida. The structure of the 

 genus Yoldia being better known, I shall confine myself to pointing out the following 

 facts : — 



The branchial siphon (inhalent) is not open ventrally. 



The siphonal tentacle (probably osphradium) is placed sometimes to the right, 

 sometimes to the left. The presence of pallial tentacles, properly so called (on the 

 posterior edge of the mantle), does not appear to be constant. 



Finally, the gill (as in other species of Yoldia already studied) is less simple than 

 that of Malletia, the lamellae being more numerous and more closely pressed together ; 

 that is to say, they already resemble in their arrangement those of Nucula. 



' Mitsukuri, he. cit., pi. xxxiv. fig. 5. 



