1904.] NATURAL SCIENCES OF PHILADELPHIA. 367 
from the cell identified provisionally as 2b”, which lies just beyond 
the median cells of the first row’’, and he adds, ‘‘I have not been able 
to determine whether any part of the second velar row arises by sub- 
division of cells of the first; if not this row may include a few of the 
third quartet (3a™ and 3b", fig. 56) at the points opposite the anterior 
turrets”. It also seems probable (Supplementary Note, page 204) that 
the cells 2b’"4, 2b’! lie outside the first velar row. Fig. 79 shows 
two large cells between the first and second velar rows, and they appear 
to represent the major portion of these cells. Smaller derivatives 
from them may join 2b” in forming the median part of the second velar 
row (V’). Conklin thus finds that the preoral velum arises from “a few 
cells of the first quartet, many of the second and possibly a few of the 
third”. I do not believe that the third quartet becomes involved 
in the preoral portion of the velum of Fiona, though doubtless cells 
from this series are closely connected with it in the stomodeal region 
and help in the formation of the postoral velum. It will be remembered 
that in Crepidula secondary mesoblast is derived from the second 
quartet, while in Fiona it is furnished by the anterior groups of the 
third, and in this process the large cells of this series, which have hith- 
erto lain well up on the sides of the gastrula, migrate over the under- 
lying mesoblastic elements and thus become far removed from the 
region where the velum first appears. The formation of secondary 
mesoderm in the most anterior second quartet group of Crepidula 
has doubtless the same effect of lessening the external area of the 
quartet in that region, while the neighboring third quartet cells would 
lie relatively higher in this form than in Fiona. So when the second 
velar row forms in Crepidula it will lie relatively lower in the second 
quartet group (2b”) and more probably involve third quartet cells, 
as Conklin states it probably does. 
Regarding the lineage of the velum of Planorbis, Holmes says that 
“the tip cell (of the anterior arm) divides as far as I can deter- 
mine, but once, and the two daughter cells become pushed apart by 
the cell 1b", which forms the median cell of the upper row. These 
cells extend to the anterior trochoblasts on either side, but in later 
stages they may sometimes be separated from them by cells which 
wedge in from below’. The anterior trochoblasts follow these cells 
posteriorly, but Holmes states that the tip cells of the lateral arms “do 
not form a part of the prototroch but enter into the formation of the 
head vesicle”. In this Planorbis differs from Fiona. Blochmann 
states that the right and left tip cells enter the velum of Neritina. 
The lower cells in the prototroch Holmes derives from the second 
