48 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
ing, but have probably been formed simultaneously by the spiculiza- 
tion of a particular area of the lophophore. In the smallest specimen 
seen, at least one third smaller than this one, the jugum was fully 
formed but was hopelessly broken in the attempt to reveal the posterior 
surface. 
In the adults,. the saddle is distinctly shown on the posterior surface 
(pi. 9, fig. 47; pi. 10, figs. 52, 54) but growth has rendered the suture 
much less conspicuous. There are four other specimens (all that I 
have) besides those figured that show this saddle just as distinctly. 
One of these is represented in figure 56 (pi. 10). It is very similar 
in its details to the one shown in figures 52 and 54 of the same plate. 
The two figured do not represent the largest stage as seen in figures 
48 and 49 (pi. 9), the posterior surface of which has not been seen, 
but the structure occurs with little modification in others which are 
fully as large. In all the adults the brace-like ridges of the saddle are 
much less conspicuous than in the young. 
Further, it will be noted that in the specimens figured the so called 
stem does not really bifurcate to form the lamellae, as usually described. 
The lamellae actually take their beginning on the posterior surface 
of this stem at the tip of the saddle, and from that point up to the point 
of separation they are well developed and each lies in exactly the same 
plane as the remainder of the lamella. Thus, instead of speaking of 
the point of bifurcation of the stem, it would be much better to refer 
to the point of separation of the lamellae. Each lamella, after com- 
pleting the circle, returns to near the point of origin, the two arching 
over the saddle and uniting above it on its anterior surface. The 
inner edges can sometimes be traced directly to their point of origin, 
but not always. The outer edges, on the other hand, always end on 
the outer slopes of the saddle just above its contiiniation with the 
primary lamellae, even in the youngest specimens. They seem to 
fuse with the saddle and cannot be traced onto the anterior surface, 
where they would be expected to be continuous with each other, under 
the assumption that the lamellae are united above the saddle to form 
a distinct plate. This is not believed, however, to weigh heavily 
against the idea that the whole of the jugum except the saddle was 
formed simultaneously or nearly so. Two explanations are possible. 
After the first formation of the plate, conditions of deposition inay 
have been so different on opposite surfaces of the jugum as to permit 
one to remain in nearly its original condition, the folds of the lopho- 
