456 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
ridges bounded by the parts of the zigzag. Two ridges are especially 
prominent; a long one on the left running along the second and third 
parts of the zigzag, counting from the anterior end, a short wide ridge 
on the right opposite to the left ridge across the suture valley. In 
dried specimens and often in life, these two ridges are often more 
pronounced than in figure 29 (pi. 47). They then suggest the idea 
that they may be homologous with the two tuberosities of C. afjinis, 
the one on the right passing across the median line somewhat as does 
the spur in figure 1 (pi. 43). In this hypothesis, the orifice of figures 1 
and 5 would represent the third angle of the zigzag of figure 29 (pi. 47), 
leaving posterior to this angle a three-limbed zigzag that is very similar 
in both species ; thus a depression of the tuberosities of figure 1 and 
extension of the zigzag and an elevation of the anterior edge of the 
annulus to form two cushions would form an annulus like that of figure 
29 (pi. 47). 
Removing the annulus and looking at its exoskeleton from the dorsal 
side (pi. 47, fig. 31), we see a long and much bent ridge corresponding 
to the zigzag suture in its course. This ridge presents four successive 
semicircular sweeps to right and left alternately, of which the most 
posterior is lost to sight in the deep depression representing the ele¬ 
vated posterior rim seen in figure 29 (pi-. 47). The anterior part of 
the ridge stands up very high from the rest and from it run off but¬ 
tresses right and left. These represent the obverse of the most de¬ 
pressed anterior part of the central areas seen in the ventral view 
(pi. 47, fig. 29). The anterior median elevation in figure 31 (pi. 47) 
is the obverse of the median anterior groove between the cushions. 
The elevation of the anterior part of the ridge is indicated in figure 
35 (pi. 47) where the two cushions and the median groove forming 
the anterior face of the annulus are represented above, while the down¬ 
hanging tube is the elevated ridge passing off on the left into the de¬ 
pressed central area seen in figure 29 (pi. 47), which is the same as the 
lateral buttress seen on the left, next the ridge, in figure 31 (pi. 47). 
A posterior view of the annulus (pi. 47, fig. 32) shows that its face 
here is very steep and high and that the median part of the zigzag 
suture extends down over the posterior rim a short distance. This 
also presents the annulus as a ring with an anterior notch. 
Upon decalcifying and clearing the exoskeleton of the annulus it 
was found that the dorsal ridge was really a long bent tube (pi. 46, 
fig. 23) which from this dorsal view presented anteriorly a wide open 
