530 FISHES CHAP. 
Baltic. Another genus, Didymaspis, has been found in the 
Lower Old Red Sandstone of Ledbury. 
Fam. 3. Tremataspidae.—The interorbital plate is free, and 
hence it is often lost in the fossils. Several species of 7remataspis 
occur in the Upper Silurian of the Isle of Oesel. 
As regards the origin and mutual relationships of the different 
families comprising the Heterostraci, it has been urged with 
great force by Dr. Traquair’ that they constitute a natural 
sequence of forms, beginning with organisms whose Elasmobranch 
ancestry is extremely probable, and leading to highly-specialised 
types, which, considered by themselves, possess little to justify 
any conclusions whatever as to their origin or kinship. The 
Coelolepidae form the starting-point, and in the light of their 
exoskeleton of dermal denticles, their derivation from some 
primitive Elasmobranch prototype seems a reasonable inference.” 
From the Coelolepids the path of specialisation through the 
Drepanaspidae and Psammosteidae to the Pteraspidae is marked 
(i.) by the basal concrescence of isolated denticles to form, first, 
numerous small polygonal plates, and then larger and _ less 
numerous plates, as the constituent elements of a characteristic 
dorsal shield, leaving, however, the denticles of the rest of the 
body to become converted into a rhombic squamation; (i1.) by 
modifications in the “ lateral fin-lobes,” which may become enclosed 
in the developing dermal armour (e.g. Drepanaspis), or cease to 
be recognisable (e.g. Pteraspis). The affinities of the Osteostraci 
are very obscure, and their inclusion with the Heterostraci in the 
same group (Ostracodermi) has hitherto rested mainly on such 
negative evidence as the supposed absence of paired limbs, jaws, 
and teeth ; in fact, it has been affirmed that “ there is absolutely 
no reason for regarding Cephalaspis as allied to Pteraspis beyond 
that the two genera occur in the same rocks.” * It is possible, 
however, that in Ateleaspis we have an annectent form, which in 
some measure combines the structural peculiarities of the two 
groups. That this singular genus belongs to the Osteostraci is 
proved by the presence of bone lacunae in its dermal hard parts, 
a conclusion which is strengthened by the apparently dorsal 
position of the orbits and the presence of a dorsal fin. On the 
1 Trans. Roy. Soc. Edinb. xxxix. p. 843 et seq. ; Rep. Brit. Ass. 1900, p. 768. 
2 See critical remarks by Smith Woodward, Geo/. Mag. vii. 1900, p. 66. 
3 Lankester, Nat. Sci. xi. 1897, p. 46. 
