ON SOME ARMOURED FISHES. 
7 
canals or tubules, save where a canal of some size passes from 
its aperture upwards into the walls of the next layer. 
In some cases these canals appear to open from the poly- 
gonal cavities of the middle layer ; in other cases, they come 
directly from their apertures on the inner surface of the shield. 
In the upper wall of the stratum of polygonal cavities, the 
canals take a horizontal direction, still very few in number, and 
of large size, receiving here branches from the cavities. The 
calcareous laminae are here arranged horizontally. If the section 
has been made transverse to the surface ridges, these appear as 
so many papilliform processes ; or if along one of the ridges, a 
continuous horizontal layer is exhibited. Into this layer, or 
into these papillae, the horizontal canals send short branches 
(one to each papilla), which give off minute tubules in every 
direction ; these arborescent tufts (PL I., Fig. 3, r) correspond to 
the vascular bushes seen, under the microscope, in the external 
layer of Cephalaspis* The laminated material is arranged 
round these tufts concentrically, the finer branches traversing it 
much in the same way as dentinal tubules traverse dentine. 
Indeed, each of the .sections of the ridges recalls very strongly 
the structure of a tooth, or of a dermal defence of a placoid 
fish. There is no trace of a 44 ganoin ” layer beyond the 
laminated material forming the ridges ; nor can any such layer 
be detected with the microscope. 
The characteristic structure of these shields appears to be — 
the absence of bone-lacunse, the paucity of vascular canals, and 
the excavation of the mid layer of the shield into large vascular 
sinuses (PI. I., Figs. 1-3, b). It was the exposure of the inner 
44 cancellated ” layer, by the decortication of the outer 44 striated 
layer,” which led Professors Kner and Romer to regard the 
pteraspidean shields from Russian Grallicia and from the Eifel 
as the remains of cuttle-bones. 
After his examination of the structure of Pteraspis , Professor 
Huxley remarks* : — 44 No one can, I think, hesitate in placing 
Pteraspis among Fishes. So far from its structure having 4 no 
parallel among fishes,’ it has absolutely no parallel in any other 
division of the animal kingdom. I have never seen any mollus- 
can or crustacean structure with which it could be for a moment 
confounded.” 
It must be borne in mind, in connection with this question, 
that no indication whatever of an internal bony skeleton, be- 
longing to these fishes, has been found ; and although in the 
other division, the Osteostraci'f — represented by the genus 
* “Quart. Journ. Geol. Socu” 1858, Vol. xiv. p. 277. 
t From dcrreov bone, and oarpaKov a shell or dermal bone ; because in 
this division the shell displays true bone-lacunae as well as vascular tubuli. 
