ORTMANN: A MONOGRAPH OF THE NAJADES OF PENNSYLVANIA 311 
after, the marsupium begins to form, and with increasing age the number of ovisacs 
increases. 
Since I have shown, that there are in the females of all species permanent 
differentiations of the marsupial gills, the question arises, whether these differences 
are observable at an early age, and I am able to report the following cases: 
The smallest specimen of any species I ever investigated, in which the sex 
was recognizable, was a specimen of Strophitus edentulus, 17 mm. long. Here 
(See Pl. LXXXVI, fig. 10), in a cross-section, the septa of the outer gills were 
decidedly more crowded and more solid than those of the inner gills. However, 
the peculiar folding of the epithelium of the water-tubes (as shown in older speci- 
mens, see Pl. LX X XVI, fig. 7b) was not yet developed. This specimen upon the 
outer surface of the shell exhibited one growth-rest; assuming that this was formed 
in the first winter, this specimen would have been in its second year.” 
Additional evidence, that in some species already in the second year the female 
structure of the gills begins to develop, is furnished by the following cases. 
Anodonta grandis gigantea Lea. Here I found a young individual, 46 mm. 
long, but with only one growth-line, distinctly possessing the female structure 
(Pl. LXXXVI, fig. 14). 
Symphynota compressa Lea. A specimen, 44 mm. long, with only one growth- 
rest had the female structure fully developed. 
Symphynota viridis (Conrad). A number of young ones, of various sizes (18 
to 31 mm. long), showed distinct traces of the female structure, and one of them 
(31 mm.) was even gravid (Pl. LXX XVI, fig. 11). 
Besides these cases, I did not find the female type of gills in the second year 
in any other species, although I examined specimens of Quadrula undulata hip- 
popea, Unio complanatus, Lampsilis ligamentina, L. multiradiata, L. ventricosa, 
and L. ovata. 
That specimens in the third year have the female structure, is shown in a 
specimen of Symphynota complanata, 35 mm. long, with two growth-lines. Here 
the difference of the septa of the outer gill became very striking, when compared 
with a male, 54 mm. long, with three growth-lines. 
2] am aware of the fact that the value of growth-rests for the determination of age has been disputed. It is, however, 
beyond doubt that during each winter a growth-line is formed. It is also established, that growth-lines may form at any 
other time, whenever unfavorable conditions prevail. But the latter are undoubtedly abnormal and irregular. In young 
specimens, we may safely assume, that the first few growth-rests, chiefly when they are regular, correspond to the stoppage 
of growth in winter. In older ones, it is more difficult to recognize the annual growth-lines, and to distinguish them from 
the accidental lines. Yet in well developed specimens they may be told apart. It may incidentally be mentioned here, that 
in shells from Lake Erie the growth-lines are often extremely regular, simply because in the lake accidental disturbance of 
the summer growth is very rare, and thus only annual lines are developed. 
