366 PROCEEDINGS OF THE ACADEMY OF [April, 
quartet cells which have lain nearest the first quartet area, and this 
row, the first appearance of which is indicated in figs. 97 and 98, will 
be designated V’, since it corresponds in general to the same cells in 
Crepidula which are designated by that term. Unfortunately the 
cells in this region have for some time presented no distinguishing 
marks, without which exact derivation is precluded by their number, 
but from their positions these lower cells probably correspond to deriva- 
tives of 2b™!, 1, 241 in the anterior group, and similar cells in the 
lateral. At a later period (fig. 101) these rows tend to unite to form 
an irregular line several cells in breadth, distinguishable only by their 
nuclei. As the stomodzal invagination progresses the velar rows 
are drawn forward and downward in that direction, and by the growth 
of the head vesicle they are also pushed acwnward laterally. It-is 
probable that elements of the second quartet which lie still lower than 
those already mentioned become involved in the preoral velar area, 
either functioning directly as ciliated velar cells or taking part in the 
development of the underlying region of the expanding velar ridge. 
At the period represented in fig. 103, two irregular rows of nuclei 
may be observed in the anterior cephalic region above the stomodeum, 
and these correspond in origin to the rows V' and V? above mentioned. 
The postoral velar area is but faintly demarkated in the preparations 
studied and crosses the ventral region just behind the stomodzum. 
The cells comprising it are doubtless, in the median region, derived 
from the third quartet, to which are added second quartet elements 
more laterally where the postoral velum joins the preoral. 
A portion of the velum does not in Fiona curve sharply toward the 
apical pole, as in the case of Crepidula, where an anterior branch is 
formed, but the whole extends backward around the head vesicle, so 
that this part corresponds in position to the posterior branch of Crepi- 
dula. This difference will be evident if a comparison is made between 
figs. 78 and 82 of Crepidula and fig. 108 of Fiona. In the latter in- 
stance it will be seen that the apical pole lies far forward from the pos- 
terior ends of the velar edge, while in Crepidula the anterior branch 
curves inward toward the apex, while the posterior branch continues 
backward around the whole head vesicle, as does the entire velum of 
Fiona. 
In Crepidula Conklin (Supplementary Note, p. 204) finds that the 
median anterior portion of the first velar row (V‘) probably arises 
from the divided tip cells of the anterior arm, while laterally this row 
is continued by the trochoblasts and cells at the ends of the lateral 
arms. ‘The second row in its mid-ventral region is probably “derived 
