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 Tremataspis 

 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 l 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. 2 

 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 ; (ii.) 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, Geol. Mag. vii. 1900, p. 66. 



3 Lankester, Nat. Sci. xi. 1897, p. 46. 



