8 G. LINDSTRÖM, A SILURIAN CYATHASPIS FROM GOTLAND. 
cuticulum and the vacuoles, no doubt for the fluid or semi- 
fluid mass which filled these (P1. II figs 1—4 b). The vacuoles 
are comparatively large and are closed polygonal cavities, 
without any distinct separation between the walls of the conti- 
guous ones. 
They are in communication with each other through 
transverse tubuli, which perforate the walls. Upwards the 
vacuoles communicate through such ducts with a network 
of larger canals (Haversian canals according to RoHon) which 
run in a direction transverse to that of the ridges. These 
canals again are connected with a stratum of other canals 
(P1. II, figs 1—4 e) which lie above them and go in a direction 
at right angles to them and parallel to the ridges of the ex- 
terior surface, just a little below them. There is one such 
longitudinal canal to each ridge. From them an immense number 
of minute ramifying tubuli (Pl. II, figs 1—4 f) issue, filling 
the uppermost stratum and ending near the surface of the 
ridges without any distinctly apparent opening. These are 
the dentine tubes, as the authors describing other Pteraspi- 
dians have called them. The canals, from which they issue, 
then are homologous to the pulpa cavity and may be called 
»pulpa canals>. 
There are thus in all five different kinds of camals tra- 
versing the exoskeleton, viz. 1, the ducts in the basal stratum, 
2 the communicating tubes between the vacuoles, 3, the 
»Haversian canais», 4, the »pulpa canals», and 5, the dentine 
tubuli. 
It is probable that the glossiness of the surface is due 
to an extremely thin covering of enamel, peer in all 
sections through its extraordinary tenuity. 
As to the ridges they are provided with one sharp, thin, 
prominent ledge (Pl. II, fig. 1—3 h), running all along one of 
their sides and wi vo such ledges opposite. The solitary 
ledge is finely crenated. A few ridges have only one ledge 
on each side, and these are both crenated (P1. IT, fig. 5). On 
the central disc of the dorsal shield the double ledges are 
directed inwards, while on the cornua they are directed out- 
wards and again change their position on the inflected side, 
where this turns rourd to the inferior surface. 
Between each pair of ridges an open groove (P1. II, figs 
1—3, i) circular in its section, runs parallel to them, in connec- 
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