8 PROCEEDINGS OF THE NATIONAL MUSEUM vol.75 



formed by the junction of the gills. It meets again, but is not fused, 

 to form the upper part of the inhalent siphon and below this point 

 is not united. The margins of both siphons are greatly thickened 

 and pigmented, bearing on their inward surface numerous tentacles. 

 In specimens preserved in alcohol the tentacles of the inhalent siphon 

 have an average length of 3 mm., while the tentacles of the exhalent 

 siphon are smaller and not so numerous. From the inhalent siphon 

 the tentacles continue on the inward margin of the mantle to a 

 median point, growing smaller and diminishing in number until the 

 last several tentacles are about 5 mm. apart. The margin from this 

 point to its junction over the anterior adductor muscle diminishes in 

 thickness and is free from tentacles. The edge of the mantle is dis- 

 tinctly pigmented along its entire course. This pigmentation is very 

 heavy at the siphons and at the posterior tentacle-bearing portion. 



GILLS AND KEPRODUCTIVE SYSTEM 



Plate 2, figs. 6, 7, and 9 



The gills are elongated with the inner lamellae continuous with 

 the visceral mass anteriorly. There are about 10 gill filaments 

 between the septa of that portion of the gills which serves as a mar- 

 supium and about 20 septa between that which does not, so that a 

 gill or a part of one with more numerous septa than other portions 

 is indicative of a marsupium and therefore of the female mussel. 

 There is a very inconspicuous longitudinal furrow passing around 

 the base of the inner gills and is caused by a very slight invagination 

 of the gill filaments. The inner or mesial gills have no such groove, 

 for the filaments are smoothly curved as they bend upon 

 themselves at the base. This furrow is present in the gills of both 

 sexes. In L. nasutus the gravid gill protrudes much beyond the 

 original base of the filaments. This condition is readily seen by the 

 projection of the bulging gills beyond the chitinous rods of the fila- 

 ments which have a clear-cut line of termination. This projection 

 beyond the filaments is not uniform, for the septa are continuous 

 with the stretched area, checking at their points of insertion the 

 bulging, and therefore giving a scalloped or beaded appearance to 

 the edge of the gill. 



In L. nasutus the posterior portion of the outer gills is specialized 

 to form the marsupium. 



The glochidia are of the bookless type. 



BEAK SCULPTURE 



Plate 2, fig. 5 



ANODONTA CATARACTA Say 



Plate 3, figs. 1 to 10 



In the genus Anodonta the male and female shells are alike, the 

 beak sculpture is coarse and the embryos fill the entire gills, forming 



