1894. PROCEEDINGS OF THE NATIONAL MUSEUM. 701 



tlie modian line, recurving to the point of attachment to the foot, thus 

 showing that the gill, if normal, would not be attached (as in so many 

 cases it is) to the siphonal septum, but to the foot. The structure of 

 tlie gill comprises large lateral branches, given out from the sides of 

 the stem at intervals (whicdi grow smaller posteriorly); below these are 

 close-set smaller longitudinal tubes extending from one end to the other, 

 below which again is a reticulate surface of cellular epithelium, from 

 which are given out the very narrow, zigzag, transverse lamelliTe 

 already referred to, hardly i^rojecting from the epithelial carpet. The 

 j)ortions corresponding to the outer lamina of each gill are more bellied 

 downward than the inner lamina, but both are otherwise alike in 

 tenuity and structure, except for the presence of the raphe on the 

 inner one. 



The oral palpi are thin, muscular, and smooth, except for wrinkles 

 radially directed toward the periphery from the mouth, but the surface 

 is not regularly striated. The palps are continuous in the median line 

 above and below and adnate to the surface of the mantle except at the 

 extreme edge above and partially to the front edge of the visceral mass 

 below. The absence of the regular channels on the gills and of stri- 

 ations on the palps leads to the inference that the ciliary action of 

 the gill surface plays a smaller part in the collection of food in this 

 form than in ordinary pelecypods. There is nothing corresponding to 

 the peculiar bifid lappet noted in JEuciroa. 



Serial stained sections of a part of this specimen, including the outer 

 limb and stem of the left ctenidiuni,the free, infolded lamina, and part 

 of the mantle lobe above and below the point from which the lamina 

 is given off inwardly, show that, in staining, the connective and 

 glandular tissue of the mantle and nephridium contract out of all pro- 

 portion to the denser tissues of the gill, foot, muscles, etc. The deli- 

 cate columnar fibers transverse to the lumen of the nephridium are 

 almost wholly lost, ruptured, or distorted out of recognition. By way 

 of compensation, however, the sections showed conclusivelj' that the 

 free lamina, though connected with the outer edge of the gill, is abso- 

 lutely distinct from it organically, and is continuous with and an undi- 

 vided part of the tissues of the mantle lobe from which it springs. 

 The ramification of the nephridium, which extends between the walls 

 of the mantle and out into the lamina, does not extend ventrally 

 between the mantle walls below the point where the lamina arises. 

 The space below this point, between the walls of the mantle, is filled 

 with connective tissue. The nephridial concretions, which are abun- 

 dant in the recesses of the gland, are apparently of two sorts. One, 

 which was noticed before the sections were made, is translucent, j)ale, 

 or brownish and stains feebly. The other sort in the fresh animal has 

 a more purple color, is more deeply embedded in the glandular epithe- 

 lium, and, in the sections, stains black. The presence of free ova in 

 the cavities of the nephridium I am unable to account for, but is unde- 



