183 
The posterior flap or lobe of the shield is thinner than the 
rest, but, on an average of cases, is as wide as the main part ; 
there is a well-marked lateral notch between the posterior 
flap and the main portion of the shield. The pedicle or stalk 
of the shield is approximately central; its position is indi- 
cated in text-fig. 1, F, by the central dark area. The red line 
of the shield is usually curved, and is more or less parallel to 
the posterior edge of the hinder lobe ; in text-fig. I it is repre- 
sented by a firm black line. The proportion existing between 
the average measurement from the centre of the red line to the 
front edge of the shield and that from the centre of the red line 
to the hind edge is about 20 to 7. 
It is a characteristic feature of the bud development of 
both Cephalodiscus and Rhabdopleura that the buccal shield 
differentiates very early ; the rapid increase in the size of the 
shield in the early stages of development of the buds of C. 
gilchristi is even more striking than it is in the species dodecalo- 
phus, nigrescens and hodgsont. In the earliest stages there is 
no posterior lobe ; when it does appear (text-fig. 1, A) it is 
much narrower than the main portion of the shield. It is only 
during the very latest stages, when the bud is ripe and ready 
to become independent, that the width of the posterior lobe 
becomes approximately equal to that of the rest of the shield, 
as it is in the adult (text-fig. 1, F). The red line appears at 
about the stage when the first two pairs of plume-axes can be 
recognised (text-fig.1,D). The lateral notch of the shield, due 
to a widening of the posterior lobe, does not become strongly 
marked until after the stage has been passed when four pairs 
of plume-axes are recognisable. In text-fig. 1, E, it will be 
noticed that the posterior flap is still comparatively narrow. 
It is rather remarkable that the shields of very old buds, 
namely, those with five or six pairs of plumes, but with imper- 
fectly-developed pinnules, are larger than the shields of adult 
polypides. A shield of such a stage would be intermediate 
in general characters between those shown in text-figs. 1, E 
and F, but would be larger than either. The explanation of 
this circumstance is probably that in the adult shield there is 
a greater development of muscular tissue present than in old 
buds, and that what appear to be fully expanded shields of 
adults are really less extended than are those of the old buds ; 
by virtue of their greater muscularity the shields of the adults 
have become fixed by the killing and preserving fluids in a 
partially contracted state. 
Around the front edge and sides of the main portion of the 
shield, extending as far back as the lateral notch, and not con- 
tinued on to the posterior lobe or flap, is a broad dark margin 
(fig. 4, plate 2). The width and extent of this margin are 
