THE NAUTILUS. 29 



an isolated cavity, which is evidently corresponding with, and de- 

 scendant from an ovum. These cylinders seem to be not homologous 

 with the "sacks" in Lampsilis and other groups, and may properly 

 be called placentae. When removed from the branchiae and sur- 

 rounded by water, they swell up, at the same time becoming more 

 translucent, and each embryo is dislodged from its cavity, evidently 

 expelled by the swelling of the surrounding substance, and the exit 

 facilitated by its softening. But each one is still hanging on the 

 cylinder, held by a short byssus thread, whose proximal end is 

 attached to the soft parts of the young, the distal to the inner lining 

 of the ovum cavity. Very probably these placentae are discharged 

 as such by the parent, with the young first enclosed, and then 

 attached for some time. 



In the other species named above, the ai-rangement is rather 

 similar; the young are attached to and held together by filaments 

 which seem to be homologous with the placentae of edentula. And 

 the same byssus has been seen in the young of marginata, coiled up 

 at the distal end. 



The embryonic young of these species as well known, are all of 

 the same type, i. e. pointed below and strongly " hooked," quite 

 different from those of other groups, a very significant character. 

 The shells of the adult show some common features, and their nacre 

 is of rather the same appearance. All these qualities combined seem 

 to prove that the several species under consideration, with some 

 nearly related forms, constitute a rather well characterized genus. 

 That the hinge of edentula is still more rudimentary than that of 

 most of the others, can be no valid argument to the contrary, and 

 also the more developed placentae are, in my opinion, of secondary 

 significance. 



3. Gonad and gravid branchice in the young and old; Parasite. 

 It has already been stated that in young individuals, two, three, or 

 possibly four years old, the gonads are not yet developed at all, and 

 at that period the shells show no distinction of sexes, even in Lamp- 

 silis. The animals seem to be asexual and, in this respect may be 

 regarded as larvae. There are very few animals, of higher or lower 

 order, showing this peculiarity in their apparently definite state, 

 except possibly some of their congeners, marine Pelecypods. When 

 the gonads commence growing, there are at first few acini developed, 

 producing small quantities of either ova or sperma. It is a ques- 

 tion, however, whether at that juvenile age the future sex of an in 





