430 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
generally elevated character of all the central part of the annulus is 
well shown from an anterior view (pi. 43, fig. 4), where the two tuber¬ 
osities are seen as elevations above the general level and the edge is 
everywhere much lower. The edge in the foreground was the hinge¬ 
like connection of the annulus to the sternal plate between the fourth 
legs. On the median plane an anterior valley-like groove rises up 
between the anterior faces of the tuberosities and ends abruptly where 
the transverse spur passes across from the left tuberosity (the right 
of figure 1) to end beneath and somewhat posterior to the other great 
tuberosity. 
The ehitinous exoskeleton above represented is dense and hard 
with a smooth surface slightly marked by wrinkle-like lines but free 
from setae. Posteriorly it is more or less calcified. In life the tuber¬ 
osities are somewhat yellowish or reddish while the posterior rim and 
promontory are white or bluish so that in a photograph taken with a 
non-orthoehromatic plate the tuberosities are dark and appear as 
depressions while the rim appears light and as if much elevated. 
In cutting out the annulus exoskeleton from a living crayfish there 
is some removal of underlying tissue which adheres to the dorsal side 
of the exoskeleton and must be removed to gain a clear view. Such 
a dorsal view of the exoskeleton merely is shown in figure 2 (pi. 43) 
which is the reverse of the same specimen shown in figure 1. 
From the outside the entire annulus plate is elevated and has a low 
bounding edge (pi. 43, fig. 1), from the inside view it is depressed 
and has an elevated bounding edge (pi. 43, fig. 2). The bottom of 
the dish-like annulus as now seen shows anteriorly two very deep pits 
that correspond to the tuberosities and posterior to these are elevated 
transverse regions corresponding to the transverse depression of the 
outside view. Thus the external protuberances and depressions are 
represented by their negatives on the dorsal aspect and the sculpturing 
of the annulus is obtained not so much by thickening of solid exoskele¬ 
ton as by a repousse in and out bending of the exoskeleton to fit, as we 
shall later see, over corresponding foldings of the living epidermis. 
But the most marked feature in the dorsal view is a bent longitudinal 
rounded ridge. This rises anteriorly just behind a median elevation 
between the pits of the tuberosities, a median elevation corresponding 
to the anterior median depression of the ventral view (pi. 43, fig. 1). 
The bent ridge then passes to the right and, enlarging, passes to the 
left, then diminishing, passes to the right, and near the median line 
