136 BULLETIN OF THE BUREAU OF FISHERIES. 



siliruE are more advanced, but they have advanced in different directions, and each has developed special 

 features of the sexual apparatus. Generally speaking, the Lojn/ijj/jnffi contain the most highly advanced 

 types, as is shown by the restriction of the marsupium to a part of the outer gill, and by the strong 

 expression of the sexual differentiation in the outer shell. Yet there are forms among the Anodontinas 

 which show extremely complex structures (Strophitus) unparalleled in any other genus, and the peculiar 

 glochidia of the A)iodontince surely mark a high stage of development. 



It is not necessary for our purpose to enter into a further discussion of the subject in 

 this place. 



CONGLUTINATION OF THE EMBRYOS. 



After extrusion of the eggs from the genital apertures, they are received into the supra- 

 branchial chambers, and thence pass, as has already been described, into the water tubes 

 of the gills, eventually filling up those portions which function as the marsupium. In 

 a short time after entering the latter the eggs usually become conglutinated into masses 

 which are molded into the exact shape of the cavity of each marsupial water tube (Lefevre 

 and Curtis, 1910b). The masses are of course separated from each other by the inter- 

 vening interlamellar junctions of the gills. 



Since it is a matter of convenience to have a word to apply to these compact masses 

 in which the eggs or embryos are held together, whether they be plate-like, club-shaped, 

 cylindrical, or of some other form, we shall employ the term conglutinate in referring 

 to them. Ortmann (191 1) has proposed the word placenta, which was introduced by 

 Sterki (1898) for the peculiar cords of Strophitus, but this is obviously misleading, as 

 there is no connection whatever between the masses and the maternal tissues. The 

 conglutinates vary greatly in different species in size and shape, and, since each is a cast 

 of the cavity of its water tube, they conform to the special conditions existing in the 

 several types of marsupium. The commonest form is that of a fiat plate, either elliptical 

 or lanceolate, being usually slightly blunter and thicker above and more pointed and 

 thinner below. Since we have already seen that the antero-posterior diameter of the 

 marsupial water tubes varies very much in different species, the thickness of the con- 

 glutinates must vary to the same extent. In Quadrula and Unio, for example, in which 

 the interlamellar junctions are set close together, the conglutinates are very thin, being 

 not more than twice the diameter of an egg in thickness; whereas in Lampsilis, with its 

 much more capacious tubes, they may be three or four times as thick. In other words, 

 just as many eggs will lie abreast in a horizontal section of the marsupium as the antero- 

 posterior diameter of the water tube will allow. 



This commoner lanceolate form of the conglutinate, differing, however, in size and 

 thickness, may be seen in the species of Quadrula, Pleurobema, Unio, and Lampsilis. In 

 figure 41, plate xi, two conglutinates of Lampsilis ligamentina are represented, one from 

 the flat side, the other on edge. An unusual form of conglutinate has been obser^^ed by us 

 in Quadrula meianevra; it is bifurcated and consists of two flat lanceolate masses which 

 are united for the upper third of their length, but free below. In those genera, however, 

 in which the form of the water tubes of the marsupium departs more widely from the 



