326 MEMOIRS OF THE CARNEGIE MUSEUM 
The male and female shells are generally quite undistinguishable, only in rare 
cases is there a slight differentiation. 
Remarks: The genera belonging here stand together in Simpson’s system, but 
associated with others, which do not belong here, and the essential characters of 
this group have been entirely overlooked. Sterki has correctly recognized their 
natural affinity, relying chiefly upon the remarkable shape of the glochidium. But, 
as we have seen, there are additional characters in the gills of the gravid female, 
which are common to all these forms, which are so peculiar, and so strongly marked, 
that there cannot be the slightest doubt, that this is a natural group. 
3. All the remaining genera form a third group, and are more closely allied 
to each other than to any one of the groups thus far discussed. This comprises 
the following of Simpson’s genera: Truncilla, Micromya, Lampsilis, Obovaria, Plagi- 
ola, Obliquaria, Cyprogenia, and Ptychobranchus. 
They all agree in that here again each ovisac is formed by only one water-tube, 
and that this remains simple. But we have here a special structure at the edge 
of the gills, which permits a bulging out of the ovisacs beyond the original edge of 
the gills, and this causes a projection of the marsupial gills beyond the original 
edge, indicated by the ends of the gill-filaments. The swelling of the marsupium 
takes place chiefly at and near the edge of the gill, and since near the base no 
swelling in a longitudinal direction is possible, the arrangement of the ovisaes 
often is fan-like, they being narrower and thinner near the base, and thicker (club- 
like) toward the edge. This necessarily causes a folding of the edge of the gill, 
which is most pronounced in one genus (Ptychobranchus), where the whole outer 
gill is used as marsupium. In all other genera it is only the posterior section of 
the outer gill, which serves as marsupium (sometimes only part of this section); 
and here the marsupium is almost kidney-shaped and forms only a single, irregular 
fold with the non-marsupial anterior part, and sometimes a smaller fold with a 
posterior non-marsupial section of the gill. The edge of the marsupial gill is 
peculiarly beaded, the beads corresponding to the protruding ovisaes, the furrows 
between them to the septa on the inside of the gill. 
The marsupium is generally much swollen, and the ovisacs are rather large, 
and, toward the edge, rather sharply defined externally; supra-anal opening always 
well separated from the anal; the connection of the mantle generally short or 
of medium length, leaving a rather large supra-anal opening; only rarely is the 
latter small, or entirely closed; branchial opening well defined in front; anterior 
to it there may be no special structures, or such may be present in the shape of 
papillz or flaps on the mantle edge; inner lamina of inner gill generally entirely 
connected with the abdominal sac, more rarely partly or almost entirely free. 
