50 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
the two lamellae at the tip of the saddle on the under surface are 
relatively much farther apart than in the adult for the width of the 
two is as great in proportion to the size of the jugum as is the entire 
width of tins secondary plate in the adult. Thus, it is possible that it 
is present but concealed in the posterior view by the great breadth of 
the combined lamellae. It must be noted, further, that in the adult 
the width of the united lamellae is reduced — not only relatively but 
actually — just ventral to the tip of the saddle, until the connection 
is a very slender one indeed. Without the presence of this secondary 
plate the jugum would be an extremely fragile structure. The juga, 
of which the posterior sides are figured, are probably from adult 
individuals but they are not from the largest known. Figure 54 (pi. 
10) belonged to a shell which had a length of about 16 or 18 milli- 
meters. In two slightly larger juga in which this plate alone is pre- 
served in a matrix, it is much more intimately fused to the lamellae 
and the sutures between the two are not nearly as distinct as in the 
figures. From this, it appears that this plate may be of later origin 
than the body of the jugum. Certainly it becomes much more impor- 
tant with the decrease in width of the united lamellae. Whether it 
originates in response to the resulting increasing demand for strength, 
or, again, is due to the shifting of the folds and fleshy parts of the lopho- 
phore with groAvth in that organ, is not evident. 
There is considerable indi^•idual variation in the shape of the second- 
ary lamellae just below their dorsal attachment. The simplest is 
shown in figures 45 and 47 (pi. 9) where the lamellae maintain their 
natural width throughout, up to the point of fusion with the saddle. 
This prevails also in the youngest specimen observed (pi. 10, fig. 51), 
although not shown in this drawing. In the specimen shown in figure 
55 (pi. 10), the lamellae are considerably narrowed below the point 
of union, as if the margin toward the dorsal surface had had a portion 
cut out of it leaving the present margin a re-entrant arc of a circle, 
A blunt angle is thus formed on the outer edge of the lamella. In the 
specimen shown in figures 48 and 49 (])1. 9), the re-entrant is not in 
the form of the arc of a circle but is a sharp angle, the apex of which 
falls just below the point of union with the saddle, leaving a small 
tooth between it and the point of union. The outer edge of the 
lamella is somewhat prolonged into an acutely pointed lobe. This 
portion of the lamellae and the plate on the anterior surface are the 
only jugal structures which vary to any degree. 
The outer margins of the lamellae of the jugum are fimbriated. 
