172 BULLETIN OF THE BUREAU OF FISHERIES. 



It has been known for a long time that in Sirophitus the embryos and glochidia 

 are embedded in short cyHndrical cords which are composed of a semitranslucent, 

 gelatinous substance, and that these cords, which are closely packed together, like chalk 

 crayons in a box, He transversely in the water tubes of the marsupium. The blunt ends 

 of the cords are seen through the thin lamella of the outer gill, which in this genus, as 

 in Anodonta and others, constitutes the marsupium. The position of the masses of 

 embryos, while contained within the gill, is so unusual that vSimpson in his "Synopsis 

 of the Naiades" established a special group, the Diagense, for Strophiius — the only 

 genus of the family in which this peculiarity exists. In other genera the embryos are 

 conglutinated more or less closely to form flat plates or cylindrical masses, each one of 

 which is contained in a separate water tube and lies vertically in the marsupium. 



So far as we are aware, Isaac Lea (1838) was the first to obsen.'e this interesting 

 arrangement which he described and figured, rather crudely to be sure, in Sirophitus 

 undulatus (Anodonta undulata). In several subsequent communications (1858, 1863) 

 he added further details and illustrations, and also mentioned the occurrence of the 

 transversely placed cords, or "sacks," as he called them, in 5. edentulus. He recorded 

 the former species as being gravid from September until March, and described the 

 extrusion of the cords from the female, as well as the remarkable emergence of the 

 glochidia from the interior of the cords after the latter have been discharged. 



The sacks were discharged into the water by the parent from day to day, for about a month in 

 the middle of winter. Eight or ten young were generally in each sack, but some were so short as 

 only to have room for one or two. Immediately when the sacks came out from between the valves of 

 the parent, most of the young were seen to be attached by the dorsal margin to the outer portion of the 

 sack, as if it were a placenta. 



The essential points in these observations have since been verified by other inves- 

 tigators. Sterki (1898), following the suggestion of Lea, has called the cords, which 

 differ strikingly from the conglutinated masses of Unio and other genera, "placentae," 

 thus indicating that he considered them to have a nutritive function. He also described 

 the extrusion of the glochidia, when placed in water, and their attachment to the cord 

 "by a short byssus thread whose proximal end is attached to the soft parts of the 

 young." He further states that the glochidia are inclosed in the placentse when the 

 latter are first discharged, and that after their extrusion they remain attached for some 

 time. 



Strophiius edentulus, which Ortmann (1909) regards as identical with undulatus, is 

 a rare species in all of the locaUties in which we have collected mussels, and, until 

 recently, our only observations on this form were made upon a few gravid individuals 

 which were taken in the Mississippi River near La Crosse, Wis., during the summer of 

 1908. Mention has already been made of our records witli reference to the breeding 

 season of Strophiius. 



After verifying the main observations of Lea and Sterki, so far as was possible at 

 that season of the year, we examined the glochidia carefully with a view to determining 

 whether their subsequent life history would exhibit any peculiarities, as might be sus- 

 pected from their relation to the cords. At that time we did not obser\^e the normal 



