REPORT ON THE MOLLUSCA. 21 



The dorsal chamber is not entirely filled b\' the visceral mass, between which and 

 the partition there is a considerable space. 



According to Dall/ Verticordia resembles in its structure the Lyonsiella {abyssi- 

 cola) described by Sars. It may be seen, however, by comparing the descriptions 

 given above, and the figures of Lyonsiella abyssicola and Verticordia tornata, that 

 there is a considerable difference between the two forms, especially in regard to 

 the gills. 



As I had a relatively large amount of material at my disposal in my researches 

 among the Anatinacea, I was able to study comparatively almost an entire group of that 

 order. In this group I studied the genus Poromya,' and I am a1)le to affirm that 

 between it and Verticordia tornata there exists a striking resemblance. 



In fact, as we shall see, the structure of Poromya granulata (PL III. fig. 7) is 

 almost exactly similar to that of "Verticordia tornata." This genus, Porom^/a, was 

 first described in its fossil state by a conchologist of my country, H. Nyst, and has 

 since been found alive ; but its organization is still little known. I shall therefore 

 describe very briefly the principal points of its structure, in order to show how much it 

 resembles that of " Verticordia tornata." 



The mantle is open almost all along its ventral surfiice, from the anterior adductor 

 muscle to (j) near the branchial aperture. The posterior apertures are surrounded by 

 a common crown of tentacles (q), rather long, and odd in number (seventeen on the 

 specimen I examined), not eighteeen or twenty, as Gwyn Jeffreys ^ says. The number 

 of these tentacles must necessarily be odd, because, as in "Verticordia" tornata, there 

 is a single dorsal one. 



The anal aperture has a short siphon (p) ; the branchial aperture has a large 

 valve inside (h), exactly similar in structure and position to that in the preceding- 

 species. 



The foot (d) is long and linguiform ; it is extended in the specimen figured. 



The mouth is surrounded with palps, just like those in "Verticordia" tornata; a 

 large anterior pair (6), and a small posterior pair (c). 



From the anterior adductor muscle (l) to the division between the two posterior 

 apertures (h) there stretches a partition (e) traversed by the foot. This partition forms 

 two chambers in the pallial cavity, a dorsal and a ventral, similar to those in the pre- 

 ceding species. On the ventral surface of the partition, on each side, are two groups 

 i9> y') of lamellae, separated by linear slits. Tliis partition is therefore disposed 

 precisely like that of "Verticordia" described above ; it has the same dorsal muscles 

 attaching it to the shell. 



' Report on the MoUusca, Dull. Mus. Onnji. /ioij'., vol. xii. p. 286. 



- 1 have to thank Prof. Anton Dohrn for an opportunity of investigating a specimen of this species. 



° Uiitibh Concholoj;y, vol. iii. p. 4o. 



