134 BULLETIN OF THE BUREAU OF FISHERIES. 



and thus the simple ovisac of Anodonta and other genera is here divided into a number of swollen, secon- 

 dary ovisacs, running transversely across the gill, each of which contains a short, more or less cylindrical 

 mass of eggs or embryos. * * * Also in Slrophilus these structures are not present in sterile females, 

 and after the discharge of the glochidia they soon disappear. 



We have obsen^ed this secondary division of the marsupial spaces in Strophitus 

 edentulus. 



We have not studied in detail the histological structure involved in the peculiar 

 differentiation of the ventral border of the marsupium of the Anodontinae and have, 

 therefore, nothing to add to Ortmann's account (1911, p. 295) of the development of elastic 

 tissue in this region, which allows of the enormous stretching of the gill in these genera 

 when gravid. The lamellae appear to separate along the mid-ventral border, especialh' 

 in the middle portion of the gill, but are here connected by an elastic membrane which 

 closes the bottom of the water tubes, with the result that "the edge of the marsupium 

 in these forms docs not appear sharp as in the Utiio group, but blunt, rounded off, or 

 truncated." This distension of the ventral edge, which is much more conspicuous in 

 some genera than in others, is evidently a device to allow of a greater expansion of the 

 marsupium. 



Lampsilincc. — It will be recalled that Ortmann includes in this subfamily Simpson's 

 HeterogeuK, Mesogenae, Ptychogense, and, although he does not refer to the genus 

 Dromus, he would probably also place the Eschatigenae here. We have already spoken 

 of the general external characteristics which distinguish the marsupia in these groups. 

 A great diversity of form is exhibited by the marsupium, but in all of the genera here 

 concerned certain features, which have been referred to, are possessed in common. 



In all of the groups here considered the marsupium is formed by a varying number 

 of specialized water tubes in the outer gill, which are modified in different ways. In 

 most, the water tubes are utilized throughout their entire length, as in Lampsilis and 

 Obliquaria, but in other genera {Cy progenia, Ptychobranchus and Dromus for example) 

 it is only the ventral portion of the tubes which retain the embryos. 



The respiratory canals, which are present during gravidity in the Anodontinae, are 

 absent in the Lampsilina;, and the entire cavity of the water tubes in the marsupial region 

 becomes filled with eggs (fig. 53, pi. xiii). The marsupium may show a high degree of 

 distension when charged, as is seen in many species of Lampsilis. It is in the Lamp- 

 silinse that we encounter the most capacious marsupial water tubes, the enlargement 

 reaching the maximum size in Obliquaria (fig. 7, pi. vii). In figure 53, plate xiii, 

 which is drawn from a gravid marsupium of Lampsilis ligamentina, the characteristic 

 appearance of the water tubes in this genus is shown. The great antero-posterior 

 diameter of the tube (w. t.) is very noticeable, as the interlamellar junctions are repeated 

 at intervals of about a dozen filaments; the relatively large size of the tubes may be 

 readily appreciated by a comparison of this figure with figures 49-52, plate xiii. The 

 interlamellar junctions, when the gill is fully charged, are stretched into thin mem- 

 branous septa (i. j.). 



The dorsal free borders of the interlamellar junctions, while not forming a closed 

 roof over the water tubes as they do in the Anodontince, in Lampsilis at least become 



