186 
OSTEOSTRACI. 
The superficial ornamentation is not very satisfactorily exhibited, 
but one specimen (P. 6109) shows an impression of the external 
surface, represented of four times the natural size in PL X. fig. 3; 
and this evidently conforms to the type described and figured by 
Lankester iu the fossils from the Ludlow Tilestones, named 
C. ornatus by Egerton (fig. 25, A). Lankester’s determination of the 
identity of the latter form with C. murchisoni is thus confirmed. 
A novel point of much general interest is elucidated by the middle 
Fig. 25. 
Cephalcispis murchisoni, Eg.— Outline of shield, and portion of ornament much 
magnified (A). After Lankester. 
layer of the shield, which is well preserved in several specimens. 
As already demonstrated by Huxley and Lankester, this layer is 
divided into a number of distinct polygonal areas, by a system of 
reticulate, branching vascular canals ; and the present specimens 
prove distinctly that it extends backwards as a pair of postero¬ 
lateral “ flaps ” beyond the rest of the shield. Such an extension 
is shown in PI. X. fig. 1, a?, in the individuals numbered I. and II. 
in the large group (P. 6023) mentioned below. The outer layer is 
broken away, so that direct continuity can be observed between the 
appendage and the middle layer, and the precise shape of most of 
the areas or plates is distinguishable. As far as the posterior ex¬ 
tremity of the shield the outer lateral margin is apparently undivided 
by vascular channels, and the areas within are approximately as 
long as broad ; but in the appendage the outer border is divided into 
