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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 in the fossils from the Ludlow Tilestones, named 

 C. ornatus by lilgerton (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. 



Cephalaspis murckisoni, 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 PL 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 



