52 ' PEOF. E. W. CLATPOLE OS" PTEEASPIDIAX FISH ES" 



YJi of an inch by the second layer, -^^ of an inch by the next, 

 and ^^- by the outermost layer (fig. ij." 



" The outer layer (a) appears to consist of a series of papillary ele- 

 vations which have a broad free end, and are attached by narrow 

 bases, so that a triangular interspace vrith its apex outwards is left 

 between every pair of elevations. The matrix filling these inter- 

 spaces, and for some distance in the immediate vicinity of the outer 

 surface, is much darker than elsewhere, and has a deep brown hue. 

 The attached ends of the elevations pass into a whitish substance, 

 which, under this . power, looks similar to their own. It is tra- 

 versed by many reddish canals, which send diverticula into the eleva- 

 tions (b) At intervals of about J^th to y^oth of an inch or 



thereabouts, thin septiform processes are given off li-om the reticular 

 layer, and pass perpendicularly inwards to the inner layer ; they 

 thus subdivide the second layer into a series of irregularly quadrate 

 spaces, correspondicg with the prisms seen in the superficial view. 



" The inner layer is, like the rest, whitish, and is traversed parallel 

 with its surface by four or five much whiter streaks, so that it ap- 

 pears to be composed of only a corresponding number of lamellae ; 

 but on allowing the light to pass through the section, it is at once 

 obvious that each of these apparent lamellae is in reality made up of 

 many of the primitive laminse which constitute the inner layer, and 

 that the bright and dull white streaks are due entirely to a differ- 

 ence of texture or composition in the successive groups of laminae." 



The resemblance between Scajjhaspis Lloydii and the Pennsyl- 

 vauian fossils can readily be seen on reading the following descrip- 

 tion of the latter. 



The shield of these fossils consists of three distinct layers, inner, 

 middle, and outer (see figs. 2—4, p. 54). The inner layer (a) consists 

 of material in which I have been unable to detect any appearance of 

 organization, unless a delicate lamination can be so called, of which 

 I have occasionally seen traces. The thickness of this layer varies 

 considerably, but is usually about g^o ^^ ^^ inch, or from ^ to i of 

 the total thickness of the shield. It forms the floor on which the 

 rest of the structure is built up, and consequently lines the whole 

 concave side of the fossil. It shows a very faint tinge of yellow in 

 almost all my specimens. Scattered over it, apparently without any 

 regularity, are a number of minute circular openings which formed 

 a means of communication between the body of the animal and the 

 substance of the shield, and through which, we may infer, passed 

 the organs of sensation and nutrition, or at least the latter. Some 

 of these openings penetrate this basal layer, and enter the cavities 

 of the cells presently to be noticed, while others pierce it opposite 

 the walls, and continue their course for some distance in them, 

 before or without communicating with the cells. These openings 

 pass through the basal layer almost or quite at right angles, but in 

 most cases a small deviation may be noticed. They are about -^^ 

 of an inch in diameter, but may be distinctly seen and counted with 

 the help of a hand lens ; among them are a few of smaller size. 

 Both surfaces of this layer are uneven, and by this means the varia- 



