ii8 



CONKLIN. 



[Vol. XIII. 



Although no description is given of the further history of 

 these cells in Neritina, Blochmann figures a much more 

 advanced stage (his Fig. 56), in which only the macromeres 

 and the " Urvelarzellen " are labelled. Although he has not 

 described the derivation of any of the twelve new micro- 

 meres in this figure, so faithfully is it drawn, that it is possible 

 by comparing it with similar stages in Crepidula to deter- 

 mine the origin of each one of these new micromeres. I have 

 reproduced this figure in Diagram 12, b, and inasmuch as its 

 cells are not labelled in the original I have simply designated 

 the cells as in Crepidula, and have indicated their origin by 

 arrows. 



It will be seen that two new cells in each quadrant lie just 

 outside the turret cells and between adjacent arms of the cross. 

 These cells correspond, I believe, to the ones which in Crepidula 

 I have called the middle right (2a'-^-', etc.) and the upper left 

 (2a^-'-% etc.). If this interpretation is correct, there are at the 

 most advanced stage in which the cells can be identified six 

 cells of the second quartette in each quadrant of Neritina. 

 These cells correspond in every respect to the six cells of this 

 quartette which are found in each quadrant in Crepidula, viz., 

 the lower, middle, and 7ipper right, and the lower, middle, and 

 tipper left. 



The following table shows the divisions of this quartette as 

 given by Blochmann, and the corresponding cells in Crepidula 

 enclosed in brackets : 



CdlbTaqe \t 



36^=T 



A^(2.a) 



a?(^''Ti))Cell) 



a^ (3d') 

 af(3d-) 



N?of^"-5 Quart Cells. 1 



Adopting the amendments which have been suggested, the 

 divisions of 2a in Neritina are as follows : 



