PHYLOGENY OF THE PELECYPODA. 369 



It is a striking fact that the embryonic shell of the completed prodissoconch period of the 

 Unionidaj has not changed in foiin or size from the condition it had when ready to leave 

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

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

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

 prodissoconch of the UnionidjB, 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 probably to be explained by the condition of parasitism 

 which exists at this period in the development of the embryo. While the prodissoconch 

 of the Unionidae is in all cases gi'anulated on the snrfjice and shows no prismatic tissue, 

 the succeeding dissoconch is characteristically prismatic. This condition is comparable 

 to what I have described in the Aviculidse 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 cleai'ly related to 

 any other family. The peculiar form, the spin's and absence of umbos in the prodisso- 

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

 embryo. Althongh it cannot he supposed to fully represent in the ontogeny of the in- 

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

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

 sentative of an early ancestor wliich was without that structm*e (the same comparison that 

 is made in the prodissoconch of the Aviculidaj) . The radical ancestor, of which this 

 prodissoconch is the inherited representative, i)robal)ly had umbos directed posteriorly, 

 for though the prodissoconch of the Unionidaj 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; l)ut is fully imi)lied by the fact that the 

 completed prodissoconch has two adductor muscles, 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 

 l^redecessoi's of the group. 



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

 in November. The adults wei'c crowded with large embryos, and as many as fourteen 

 were taken from the gills and mantle cavity of the parent. The embryos, PL xxx, fig. 

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

 crawl actively when removed fi'om the parent. The foot has a prominent otocyst as fig- 

 ured 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 Pisidium 

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

 significance. The shell of Sphserium, Pi. xxx, fig. 4, represents an incompleted prodis- 

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



