8 OLD RED SANDSTONE EISHES. 



{C.ornatus, C. Balweyi), the outer surface, if well preserved, and not broken away as it usually 

 is in C. Lyellii, shows a continuous polished surface ornamented with tubercles and papillae 

 variously disposed, without any indication of a tesselated structure. In all species of 

 Ceplialaspis, in Auchenaspis, and Thyestes, this tuberculate ornamentation on a continuous 

 polished surface is an unmistakeable naked-eye character of the shield. As regards 

 the internal surface in Cephalaspis Lyellii and the allied species, it never exhibits any trace, 

 when properly preserved, of the apparent tesselated structure seen when the external surface 

 has been to a certain extent removed. It appears smooth, polished, and enamel-like, except- 

 ing where it is furrowed by numerous shallow depressions which radiate from the region of 

 the orbits and occiput towards the margin, before reaching which they repeatedly sub- 

 divide and anastomose. There can be little doubt. Professor Huxley observes, that these 

 are the impressions of the vessels which ramified under the disc during life. These 

 radiating channels leave their impress in the convex coats of the interior of the shield, and 

 it was their occurrence which led Professor Agassiz to speak of " fibrous bones." In 

 those forms allied to C. Lyellii, but differing in having the shield divided antero-posteriorly 

 into two plates, the anterior plate alone shows these radiating channels ; and, indeed, they 

 are often deficient in the posterior part of the shield of true Cephalaspids. The inner 

 surface of the shield in Thyestes is not known. This radiated channeling of the internal 

 surface of the shield may therefore be taken as a constant character in Cephalaspis 

 Lyellii and its congeners. 



When a vertical section is made through the shield in CepJialaspis Lyellii, it is seen 

 to be exceedingly thin, scarcely anywhere exceeding the ^^th of an inch in thickness, 

 excepting at the margins and spine ; it appears to the naked eye to consist of a compact 

 white calcareous substance. The shield in C. Salweyi, C. asterolepis, and others, is not so 

 thin, and the posterior plates of those species which possess them are also much thicker 

 than the anterior orbit -bearing portions. 



If the section of C. Lyellii be now prepared and placed beneath the microscope, the 

 appearances thus described by Professor Huxley^ as visible in a section near the spine 

 are observable^ : — The section " presents three regions or layers, distinguishable from one 

 another partly by their minute structure, and partly by the different mode of distribution 

 of the vascular canals by which the tissue is permeated in each. The innermost or deep 

 layer is made up of superimposed lamellae, not more than 50'ooth of an inch thick, each of 

 which sometimes appeared to be still more finely laminated. Interspersed among these, 

 at greater or less distance, are numerous osseous lacunae, whose long axes are parallel 

 with the planes of the laminae. The length of these lacunae varies greatly, but may be 

 taken at a^'o^th of an inch on the average ; some, however, are twice or three times this 

 length, while others are much less. The transverse diameter is equally variable ; but 



^ ' Quart. Jour. Geol. Soc.,' vol. xiv, p. 271, &c. 



- The illustrations of the microscopic structure of the Osteostraci will appear in the next portion of 

 this Monograph, which will treat of that Section. 



