AGNATHA. 
55 
are the Coelolepidae (“ hollow scales ”), which seem to have 
flourished earliest and must have been especially abundant 
in late Silurian seas. Their skin-tubercles were never united 
into plates, and when these little bodies were originally 
discovered in immense numbers in the Ludlow Bone-bed at 
the top of the Silurian formation on the borders of Shrop¬ 
shire (Table-case A) they were naturally mistaken for the 
placoid scales of sharks. It is only under exceptional 
circumstances that the animals covered with so incoherent an 
armour could be preserved intact, but good specimens have 
been found in the Upper Silurian and Downtonian shales of 
southern Scotland (Table-case A). In Thelodus the tubercles 
are quadrangular and flattened, while in Lanarhia (Fig. 48) 
Fig. 48. —Restoration of Thelodus scoticus , from the Downtonian of 
Lanarkshire; about one-half nat. size. The head is shown from 
above, the tail twisted, to be seen mainly in side-view. (After R. H. 
Traquair. Table-case A.) 
they are conical prickles of variable size. The internal 
skeleton is never preserved. 
In the Pteraspidse the skin-tubercles are united into 
plates and scales, and form fine enamelled ridges concentric 
with the edges. This family also flourished in the Upper 
Silurian, but is commoner and better developed in the Lower 
Devonian. Cyathaspis (Fig. 49) has been found in the 
Wenlock rocks of Gothland and in the Upper Silurian both 
of Europe and North America. It is the oldest fish-like 
organism of which there is any definite knowledge. Pteraspis 
(Figs. 50, 51) is a typical Lower Devonian genus, and 
numerous specimens are exhibited from the Lower Old Bed 
Sandstone of the Welsh border. 
The latest Heterostraci, which range from the Lower to 
the Upper Devonian, comprise some relatively large species, 
perhaps 2 feet in length, and form the family Psammos- 
teidse (Table-case B). The skin-tubercles fuse into small 
Table- 
A. 
Table- 
A. 
Table- 
B. 
