1894. PBOCEEBINGS OF THE NATIONAL MUSEUM. 695 



points of strnctnre wliioh were not observable otherwise. It is 

 evident that both methods are required for complete results. In the 

 present instance, in examining the gills in water with low power"^, it 

 was observed that the close set oblique plates, or lamella?, are connected 

 at their dorsal edges by a delicate series of connective fibers running 

 in an antero posterior direction and recalling the threads which connect 

 the dorsal edges of the laminoe in Poromya, but more numerous, and 

 laterally, near the attachment to the mantle, forming a sort of fascia, or 

 layer of fibres. Beside this, the dorsal portion of the plates near the 

 arterial stem of the gill show a few reticulations carrying blood vessels, 

 and a good many which appeared purely fibrous. The vascular con- 

 nectives, except close to the stem as above mentioned, were not observed, 

 though here and there a fibrous link nnited the faces of two plates 

 near their dorsal margins, but without any regularity of situation or 

 succession. The arterial stem, which anteriorly has a roughly trian- 

 gular section, near the i)Osterior ends of the gills is produced vertically, 

 so that the short laterally extended plates of this part of the gill, instead 

 of hanging below the stem, are projected from its oj)posite sides, and 

 are not all of the same vertical width. This ai)peared very clearly in 

 the microtomic sections, in which, however, no trace of the longitudinal 

 dorsal fibers could be recognized, the latter having been apparently 

 destroyed by the contraction induced by chemical treatment nsed in 

 staining, with many other more delicate features. The sections there- 

 fore show the lamellai as more isolated than they are in reality, except 

 near their ventral edges, where they are bordered by a narrow band of 

 giant cilia, which interlock between the plates, thns holding the ventral 

 margins quite firmly together. These junctions were well shown in the 

 sections, and also, though less clearly, the distal margins of the plates 

 showed patches of smaller cilia, not continuous with the band above, 

 but projecting into the peripedal cavity, and doubtless serving, as in 

 other pelecypods, the purpose of collecting and propelling grains of 

 edible matter toward the mouth. 



The nei^hridia lie below the pericardium and are distinctly limited 

 by the connective tissue made u]) of a radial network of fibers which 

 constitutes the lamina to which the outer edges of the gills are attached. 

 The nephridia have a common cavity (tig. 1, iv) more or less occupied by 

 thin folds of very delicate tissue of a more or less glandular nature, upon 

 and around which are clustered large numbers of spherical nucleated 

 or concentric concretions similar to those already described in Lyonaiella 

 and other Anatinacea. These concretions stain deeply and are very 

 varied in size, the largest exceeding any of those noticed in Halicardia. 

 The nephridia do not extend laterally into the lobes of the mantle as 

 they do in Halicardia. 



The character of the gills above described is such that it seems not 

 unreasonable to regard them as intermediate between the foliobranchiate 

 gills of such a mollusk as Solemya and the plicated reticulate gills of a 

 more modern type of bivalves, such as Lyonsiella or Halicardia, They 



