MUSEUM OF COMPARATIVE ZOOLOGY. 49 
branches were possessed of fewer generations than the intermediate ones. 
Since by Rule 2 the lateral branches were given off towards the axils, | 
and the ancestral branches therefore always remained marginal, it re- | 
sulted that the ancestral branches were the shorter, the lateral branches | 
the longer. But in Lepralia lateral branches are turned away from the 
axils, and here we find the conditions concerning the relative number of 
generations in marginal and intermediate rows correspondingly reversed. 
Thus, in Figure 71°, the terminal individual D of row 10, a median but 
ancestral row, belongs to generation IV. while the lateral branches 6 
and 15 have five generations of polypides. Thus it is true here, as in 
Bugula, that the ancestral branches are the shorter, the lateral branches 
the longer (page 44), 
That the outer individuals a, a, of rows 6 and 15, have given rise to 
more individuals than the inner C, is clear without further comment. 
Finally, since the individuals retain a nearly constant width, the neces- 
sity of the rule established for Bugula, — viz. that there is almost ex- 
actly the same number of individuals per unit of, are for all radii, — and 
of its corollary, — that the increase of individuals in sucessive gener- 
ations undergoes a regular diminution, — is apparent. 
Plustrella hispida, Fabricius.’ — This stock is a very dense corm-like 
one. The primary individual becomes surrounded on all sides by the 
younger zooecia. It is very evident from an inspection of the position 
of this primary polypide with relation to the periphery, that growth 
occurs most rapidly on each side and in front of the primary polypide. 
In making any diagram of such a stock, it is not very difficult to decide 
upon the origin of the more peripheral individuals of the stock, but it is 
wellnigh impossible to say with any certainty what are the relations 
of the individuals of the second generation to those of the first. Bar- 
rois (77, pp. 227-229) has, however, determined this for this species, 
and my diagram (Fig. 67) is based in part upon his observations. I 
do not desire to insist that the diagram represents the exact method 
of growth of the stock. It is an attempt to represent it, founded princi- 
pally on careful study of Figure 69. The quincunx arrangement of in- 
dividuals is already apparent in the young stock (Fig. 69) ; it becomes 
1 Hincks (’80, pp. 504-506) makes the existence of a larval bivalve shell a char- 
acteristic of this genus, and therefore I assign to it a very common Aleyonidium-like 
form which was extremely abundant on Fucus at Newport. F. hispida is the only 
species of this genus. I found the bivalve shell still adhering to the primary indi- 
vidual of a young colony (Plate VIII. Fig. 69, 0.). In Verrill’s (78, p. 708) catalogue 
this species is referred to under the name “ Alcyonidium hispidum, Smitt.” 
VOL. XXII — NO. 1. 4 
