122 BULLETIN OF THE BUREAU OF FISHERIES. 



In Pleurobema cesopus the eggs and embryos, like those of Qimdrula ebena and trigona,. 

 are usually, but not always, colored red or pink, but the glochidia are invariably unpig- 

 mented. Unfertilized eggs in varying proportions are frequently found in this species 

 either mixed in with embryos at all stages of development or occurring alone; such eggs 

 always show a definite stratification of the egg substances. 



DiageticE.— This group was established by Simpson to receive the genus Strophitus, 

 in which the marsupiura occupies the entire outer gill and in external appearance is 

 similar to that of the Homogenae. But it is unique among the Unionidae in that the 

 embryos and glochidia are embedded in gelatinous cords (called "placentae" by Sterki, 

 " placentula; " by Ortmann), which lie transversely in the gills, whereas in all other cases 

 the egg masses are placed vertically, each one occupying an entire water tube. In 

 Strophitus, on the other hand, the cords are packed closely together, like chalk crayons 

 in a box, a variable number being contained in a single water tube, while the blunt ends 

 of the cords are distinctly seen through the transparent external lamella of the outer 

 gill. It should be stated that Ortmann (1910b, 191 1) has found that the discharge of 

 the cords is not through the lamellae of the gills, as Simpson (1900) has maintained, but 

 that it occurs in the usual manner through the supra-branchial chambers. A description 

 of the unique cords and the extraordinarily interesting life history of Strophitus is reser\'ed 

 for a special section. 



HeterogencB.—ln this group the marsupium occupies only the posterior portion of 

 each outer gill, varying in extent from about one-third to two-thirds of the entire length 

 of the latter. In young females the marsupium is shorter and not so fully distended as 

 in older ones. In fact, it is true of all Unionidae that the marsupium is less heavily 

 charged when the female is young. The differentiation of the posterior region is very 

 conspicuous even in the non-gravid female, as the marsupium is sharply marked off 

 either by a distinct fold or a notch from the anterior respiratory part, and, since it is much 

 deeper dorso-ventrally than the latter, it projects farther down into the mantle-chamber. 

 Its walls are also more membranous in appearance than are those of the respiratory 

 region, and after the discharge of the glochidia it is seen as a flabby collapsed pouch. 



When gravid, the marsupium may be enormously swollen, the expansion being 

 greater along the ventral border than above, where, owing to its fixed position, it is inca- 

 pable of stretching. This greater ventral extension often causes the marsupium not only 

 to assume a fan-shaped form, which is so characteristic an appearance in Lampsilis, 

 but also to project forward under the respiratory portion, which in consequence becomes 

 sharply folded over on the outer surface of the marsupium. Not only is the marsupium 

 as a whole expanded in the way described, but each of its swollen water tubes is dis- 

 tended distally beyond the lower extremity of the interlamellar junctions so that the 

 ventral bordey becomes fluted or corrugated, as shown in figure 2, plate vi. This figure, 

 which illustrates the typical condition in the genus Lampsilis, is drawn from a gravid 

 female of L. subrostrata when fully charged with glochidia. The folded respiratory por- 

 tion of the gill, the fan-like expansion of the marsupium, and the corrugated border are 

 all clearly seen. 



