PHYLOGENY OF THE PELECYrODA. 369 



It is a striking fact that the embi-yonic shell of the completed prodissoconch period of the 

 Unioiiidffi has not changed in foi'm or size from the condition it had when ready to leave 

 the gills of the parents. This fact and the correlated fact that innbos have not devel- 

 oped, althongh they exist in the prodissoconch of the twenty-six other genera described 

 ill this paper, are proof of the extreme reduction of the embryonic shell. The completed 

 prodissoconch of the Unionidfe, therefore, has not progressed in the features of the shell 

 beyond the phylembryonic stage, which is found only in early embryonic stages of other 

 genera. This high reduction is jirobably to be explained by the condition of parasitism 

 which exists at this pei'iod in the development of the embryo. While the prodissoconch 

 of the Unionidae is in all cases granulated on the surface and shows no prismatic tissue, 

 the succeeding dissoconch is characteristically prismatic. This condition is comparable 

 to what I have described in the Aviculidge and their allies, but differs from what I have 

 found in other genera. 



"Wliat the affinities of the Unionidae are is a puzzle, for they are not clearly related to 

 any other family. The peculiar form, the spurs and alisence of umbos in the prodisso- 

 conch are special features, apparently due to the peculiar environment of the developing 

 embryo. Although it cannot be supposed to fully represent in the ontogeny of the in- 

 dividual any ancient ancestral form, yet certain data can be gathered from this early shell. 

 As the prodissoconch of the Unionidfe has no prismatic tissue, it is doubtless the repre- 

 sentative of an early ancestor which was without that stnictm-e (the same comparison that 

 is made in the prodissoconch of the Aviculidte). The radical ancestor, of wiiich this 

 prodissoconch is the inherited representative, probably had umbos directed posteriorly, 

 for though the prodissoconch of the Unionidse has no umbos, the more rapid growth on 

 the anterior border would have pushed umbos posteriorly had they existed. That this 

 ancestor was dimyarian might be conjectured; but is fully implied by the fact that the 

 completed prodissoconch has two adductor nuiscles, for Balfour's and Schmidt's figures 

 of a mature glochidium of Anodon from a fish have two adductors. The nepionic stages 

 of the Unionidae, though peculiar, do not so far as I am aware indicate kinship with early 

 predecessors of the group. 



Sphcerium secaris, Adams. This species was collected in a culvert at Medford, Mass., 

 in November. The adults were crowded with large embryos, and as many as fourteen 

 were taken from the gills and mantle cavity of the pai-ent. The embryos, PL sxx, fig. 

 4, show no traces of a velum as far as discerned, but have a well-developed foot and 

 crawl actively when removed from the parent. The foot has a jirominent otocyst as fig- 

 ui'ed in Cyclas by Ziegler and other investigators ; but no byssal gland was observed as 

 described in that genus. Two adductor muscles, palps and filamentous gills exist. The 

 mantle border is produced posteriorly in a single siphon, fig. 4, but is free throughout 

 the rest of its exposed margin. In older specimens and the adult, two siphons exist, 

 an excurrent and an incurrent. This was not seen in the young of the age under con- 

 sideration, but as the point was not in mind when studying the specimens, it is possible 

 that two might be found. If not, it is an interesting character, for the adult of Pisidiiim 

 has but one and the existence of one in young Sphserium would appear to be of genetic 

 significance. The shell of Spha^rium, PI. xxx, fig. 4, repi-escnts an incompleted prodis- 

 soconch stage. In PI. xxx, fig. 5, also from the gills of an adult, a completed prodisso- 



