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ole MEMOIRS OF THE CARNEGIE MUSEUM 
In Quadrula undulata hippopea, I found a specimen distinctly of the female 
type, which was 36 mm. long, and had two growth-lines; here again the female 
structure became very evident, after a comparison was made with a male specimen, 
34 mm. long, also possessing two growth-lines. 
I have an individual of Lampsilis nasuta, 38 mm. long, with two growth-lines, 
which is gravid. 
That absolute size is no proper criterion in this question, is shown, for instance, 
in the case of Lampsilis iris, of which I have a specimen 33 mm. long (smaller 
than in the instance just mentioned) which is gravid, but has at least three distinct 
. growth-lines, and consequently would be at least in its fourth year. 
All other females observed, sterile or gravid, had at least three growth-rests, 
and should be considered as being in their fourth year of age, or older. 
Thus it becomes apparent, that the female structure, at least in some species, 
develops as early as the second year, and that one at least (Symphynota viridis) 
may already become sexually mature at this time. In others, the female structure 
is distinctly seen in the third year, and some of them (Lampsilis nasuta) may 
bring forth progeny. Later, from the fourth year onward, the time of maturity 
is reached in most of the species. 
F,. DirFrERENTIATING STRUCTURES OF THE EDGE OF THE MANTLE. 
Simpson in the diagnosis of a number of genera states that the edge of the 
mantle is “double.” According to my observations, it is double along its whole 
extent in all forms. While the outer edge is closely adjacent to the margin of the 
hard shell, and is always simple, the inner edge, which is generally parallel and 
close to the outer edge, is more or less expansile, and is able to fill and close the 
gap between the two halves of the shell, when the adductor muscles relax, that is 
to say, when the shell “‘opens.’’ And further, this inner edge of the mantle is not 
uniform in its structure all along its margin, but shows certain differentiating 
structures, some of which are systematically valuable. 
1. The anal and branchial openings (‘siphons’). 
The part of the edge of the mantle at and near the posterior end of the shell 
is modified so as to form what is known as the anal and branchial openings, the 
former lying more dorsally, and immediately above the latter. They are formed 
by the mutual approach of the edges of the inner mantle at certain points, and 
sometimes by their coalescence. The two openings are separated from one another 
by the diaphragm, discussed above (See p. 288), composed of the posterior ends of 
