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^2*, *^^, 2" 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 stomodaeal 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 downward 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 stomodseum, 

 and these correspond in origin to the rows V^ and V- above mentioned . 

 The postoral velar area is but faintly demarkatecl in the preparations 

 studied and crosses the ventral region just behind the stomodaeum. 

 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 



