92 THE NAUTILUS. 



testes with the white, creamy sperma (rarely of a different color.) 

 During early summer, the branchise 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 sacs 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, 2 

 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. lev* 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. circu/us 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 runeatus 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, c<i<vi>i<'un etc. ; the shell is very 



2 More about this foatiire will be said in another place. 



