1858.] HUXLEY — CEPHALASPIS AND PTERASPIS. 2/1 



wall on each side of one of these depressions, the latter may become 

 almost converted into a canal, so as to retain a portion of the matrix. 

 This however is a rare occurrence. 



When the concave inner surface of a disk and the convex cast of 

 another specimen are compared, it is at once seen that the " radiating 

 fibres" of the one correspond with the grooves and furrows of the 

 other. The surface of the cast is remarkably darker than the sur- 

 rounding matrix, and might not unreasonably at first be supposed 

 to be of a different nature. When the inner surface of the disk is 

 carefully examined with a magnifying glass, a number of reddish- 

 brown minute dots appear scattered irregularly over its surface. It 

 will be seen immediately that these are the internal openings of vas- 

 cular canals which enter the substance of the disk. 



If a vertical section of the cephalic shield of Cephalaspis Lyellii 

 is carried through the orbits and perpendicularly to the axis of the 

 body, it will be seen that the disk is exceedingly thin, hardly anywhere 

 attaining ^-th of an inch in thickness, except at the margins and the 

 spine, which are thicker. At the lateral margin the thin lamella is 

 bent abruptly and almost horizontally inwards for about a quarter of 

 an inch. It then suddenly thins so much as to be little more than 

 a flexible membrane, which in the specimen now under description 

 is pressed up into close proximity with the dorsal part of the shield 



The thinness and fragility of the disk of Cephalaspis render it 

 difficult to obtain good sections for microscopical examination. The 

 best I have seen (PL XIV. fig. 1.) is taken at an angle of about 45° 

 to the longitudinal axis of the head, and intersects the occipital spine 

 just beyond its origin. The section of the spine is in the best con- 

 dition, and may be described first. 



It is about -j^th of an inch thick in its thickest part, which corre- 

 sponds with the median ridge of the spine, and presents three regions 

 or layers, distinguishable from one another partly by their minute 

 structure, and partly by the different mode of distribution of the vas- 

 cular canals by which the tissue is permeated in each. The inner- 

 most or deep layer (d) is made up of superimposed lamellae not 

 more than -g^nnfth of an inch thick, each of which sometimes ap- 

 peared to be still more finely laminated. 



Interspersed among these, at greater or less distances, are numerous 

 osseous lacunae, whose long axes are parallel with the planes of the 

 laminae (fig. 3). The length of these lacunae varies greatly, but 

 may be taken at J^tli 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 nOne that I measured ex- 

 ceeded -g-^-^th of an inch in this direction. The form of the lacunae 

 is very irregular in consequence of the long branching and anasto- 

 mosing canaliculi which are given off not only from their ends but 

 from their sides. In some parts the innermost layer appears almost 

 black when viewed by transmitted light, in consequence of the quan- 

 tity of air retained in the multitudinous lacunae and canaliculi. 3 



l>nx?$ vascular canals, measuring from -g-oiyth tp ^yth of an inch 



