MUSEUM OF COMPAKATIVE ZOOLOGY. 



47 



generation XXIV., reaches 3 maxima of XXVI., XXVIL, and XXVIII. 



respectively, and falls again at the other margin to generation XXIII. 

 In the subfan from which Figure 6Q was taken, the curve begins at the 

 outer margin with generation XVII., rises to generation XXII. at two 

 points, and falls again to XX. at the inner margin of the subfan. 



Of the four proximal individuals in any fan here, as in Bugula turrita, 

 the outermost, ancestral give rise to the greater number of individuals. 

 In one case, for instance, the marginal individuals lie at the base of 31 

 rows with 184 individuals, while the inner ones support only 7 branches 

 with 65 individuals. Similar results were obtained from other stocks. 



With the middle of the primary individual as a centre, I passed art 

 arc of a circle through the extremities of the branches of a large camera 

 drawing of a fan of B. flabellata, divided the radius into eighths, and 

 passed arcs through these points. The number of individuals cut by the: 

 different arcs was then counted and tabulated ; the arc with the longest 

 radius cut through 87 individuals. By measuring the length of the arcs, 

 the number which should be cut by each arc on the assumption that the 

 number of individuals per unit of arc is constant for all radii was deter- 

 mined. This was then compared with the actual number found, with, 

 the following results : — 



In this instance, then, the 7th rule deduced for B. tun-ita evidently 

 holds true for B. flabellata. 



While at Mr. Agassiz's laboratory at Newport, during the summer of 

 1890, I had frequent opportunity to examine other stocks of Bryozoa, 

 which occur there very abundantly. I will take four species as typical 

 examples of the gi-oups they represent, and treat of the architecture of 

 their colonies. 



Lepralia Pallasiana, Busk.^ — It is not at all easy to determine 



1 I do not feel perfectly certain that the specimen shown in Figure 71 (Plate 

 VIII.) belongs to this species, because the characters of the young stocks differ some- 



