62 
short description, pp. 2 and 3, and a few figures, Plate I, 
Figures 4, 5, 6, 7, 8, 16 and 17, of the process of segmenta- 
tion. His observations are very fragmentary and unsatis- 
' factory, but they would seem to indicate that the segmentation 
is total and regular, and not at all like that of the oyster. 
Granin reaches a similar conclusion, and says (Beitrag zur 
Lehre von den embryonalen Blatter bei den Mollusken. Abst. 
in Hoffman u. Schwalbe’s Jahresberichte. 1. 1872), that in 
Cyclas the segmentation is total and regular, and results in 
the formation of a spherical layer of similar cells around a 
central cavity. 
Von Jhbring and Bab!, on thb other hand, give observa- 
tions which indicate that the segmentation is, on the whole, 
like that of the oyster. 
Von Jhering says (Ueber die Ontogenie von Gyclas und die 
Homologie der Keimblatter bei den Mollusken, Zeit. f. Wiss. 
Zool. 1876- xxvii), that although he did not succeed in getting 
as complete a series of forms as Flemming has figured in An- 
odonta, the stages which he has found show that the process 
of segmentation takes place about as it does in Anodonta, and 
Babl says, p. 340, that he has observed two stages in the seg- 
mentation of Cyclas, and that the mode of segmentation is the 
same here as in Unio. In Taf. XII, Fig. 58, he shows one of 
these stages, which differs from one of the later stages of the 
- segmentation of the oyster egg only in the presence of a large 
segmentation cavity. 
These references, which cover the whole field of our exact 
knowledge of lamellibranch segmentation, show that proba- 
bly in the Cycladidse, and certainly in Unio, Anodonta, Cre- 
nella and Cardium, we have nearly the same mode of segmen- 
tation as in the oyster ; but that the normal method of oyster 
segmentation is indirect, and maybe simplified occasionally 
in the oyster, and normally in Unio and Anodonta, by the 
omission of many of the stages of the process and the reten- 
tion of those only which lie in the direct line of development. 
I have described this process in the oyster with great minute- 
ness, and perhaps with tedious exactness, since I believe that it 
