299 
irregularities here also often may be obvious. — The largest plates 
of the wall are carina and rostrum. 
The thick chitinal cuticle is dark chocolate-brownish, but worn 
off on top of the compartments, which are of dirty whitish colour. 
_ The compartments are symmetrically shaped, their greater 
part hidden below the outer plates. 
The lower part is longitudinally regu- 
larly ridged in the same way as the 
carina (Fig. 45 b); the ridges are not 
seen internally. The basal margin is 
crenulated, or rather finely denticul- 
ated. In the present specimens the 
prominent part of the compartments 
is strongly corroded all over and al¬ 
most flat. Probably the plates in young 
specimens will turn out to be almost 
conical with convergent outer ridges. 
The opercular plates externally ex- 
hibit very deep ridges of growth, but 
their upper parts are strongly corrod¬ 
ed ; the plates are astonishingly thick. 
The s c u t u m (Fig. 46) has a straight 
occludent margin; no sulcus is present. 
The articular ridge is very prominent, with a very deep articular 
furrow below, and a somewhat shallower one above. The internal 
surface is deeply excavated, with a pit for the adductor, but no 
crests. 
Tergum has a very prominent articular ridge, and a deep art¬ 
icular furrow, but no spur. The inside is deeply excavated with 
several crests for the depressor. 
None of the specimens are intact; in all of them the basal part 
is wanting, and it must thus remain unsettled whether the basis 
is calcareous as in Catophragmus imbricatus Sowerb. or mem- 
braneous as in Catophragmus polymerus Darwin. The largest spec- 
imen has a greater diameter of 55 mm, but the position of the 
opening seems to indicate that the width of the entire animal must 
have been about 70 mm or even more. It is thus by far the 
largest Catophragmus hitherto known. 
Fig.46. Catophragmus Pilsbryi from 
Taboga, Panama, a tergum, ex- 
ternal side, b seutum, external 
side, c tergum, inside, d seutum, 
inside. [X 2,7]. 
