92 THE NAUTILUS. 
testes with the white, creamy sperma (rarely of a different color.) 
During early summer, the branchiz will be found filled first with 
developing ova and then with embryones, which are discharged 
about in July; at that season the ovaries and testes are sterile. 
There are slight variations as to the time, but the general rule is as 
stated. 
This radical difference in the season of producing ova and sperma, 
and maturing the embryones, coincident with the differences of the 
location and formation of the uterus saes is highly interesting, the 
more so as they are in harmony with the differences in the shells, 
and thus seem to furnish systematic characters of a high order. In 
group A the mussels are generally more or less elongated, or at least 
longitudinal, that means with a small angle of torsion of the axis,’ 
the hinge-teeth are moderately strong, and the epiconch is of a 
vivid color, as a rule, with numerous rays. In B, the mussels are, 
as a rule, shorter, the axis-torsion is more considerable, the hinge 
and hinge-teeth are stronger and of a different shape, and the epi- 
conch is generally of a more uniform, dusky color. 
Yet there seem to be some real or apparent exceptions, and dissi- 
dent members of both groups. In U. lens Lea and ellipsis Lea the 
propagating organs are of exactly the same type as in group A, and 
also bound to the same season. In the former, which has often been 
mistaken for U. cireulus Lea, the male and female mussels are very 
different, the latter being strongly dilated downward posteriorly— 
Lea’s figure represents a female specimen well; the shell is also not 
heavy. In U. ellipsis the female mussel is also dilated and more 
full posteriorly, though not so marked, yet, as a rule, recognizable ; 
the lighter color and green rays of the younger approaches it to 
group A, and more so, in the writer’s opinion, the formation of the 
posterior mantle edge, which is of the same appearance as in the 
other species ranging under A. 
On the other hand, two species, with an elongated mussel, U- 
cuneatus Barn. (niger Rat., erassidens v. C. Lam.) and gibbosus: 
Barn. range with group B, as to their generative organs, and also in. 
the season, while they at least show no downward dilatation of the 
female shell (in cuneatus it seems to be decidedly higher in general) 
and their shells are comparatively heavy, the epiconch is of a uni- 
form deep brown or black, though having some green rays when 
young, as do also U. subrotundus, coecineus ete.; the shell is very 
2 More about this feature will be said in another place. 
