BIIIANG TILL K. SV. VET.-AKAD. HANDL. BAND 2(). AFI). IV. NIO 10. 9 



nearly eqnal to the width of the ventral la min a, while it 

 gradually becoraes less deepened behiiid. The small bevelling 

 of the postero-interiial angle (m) of the ventral lamina, proves 

 the diminutive proportions of the median ventral plate. The 

 triangulär area (2') at the hinder börder is the iisual surface 

 to overlap the posterior ventro-lateral plate. As in other 

 specimens, there is a nearly regular margin of stain ronnd the 

 plate, which might easily be mistaken for an overlapping surface. 



No other plates in the collection can be identified with 

 certainty. In fact, nearly all the specimens represent one or 

 other of the three elements just described. We therefore ob- 

 serve a phenomenon here which has already been noted in 

 Devonian sandstones elsewhere in the case of Bofhriolcpis 

 and Pteraspis] namely, that where one example of a parti- 

 cular plate occurs, others of the same kind are scattered round 

 it. In the Catskill Formation, for instance, the anterior me- 

 dian dorsal is the commonest plate of Bothriolepis ; while in 

 the Lower Old Red Sandstone of the west of England, the 

 dorsal shields of Fteraspis occur exclusively in some groups, 

 whereas the ventral shields only are met with in others.^ 

 It appears as if the plates of armour, when separated, had 

 drifted about with currents and been sorted in accordance 

 with their shape and size. 



For the generic determination of the fossils now described, 

 the anterior median dorsal plate is the most important. If 

 I am correct in assuming (I) that it overlaps both the an- 

 terior dorso-lateral and the posterior dorso-lateral elements, 

 and (II) that it is not traversed by a sensory canal, there 

 is no reason for excluding the species to which it belongs 

 from the genus Ästerolejns of Eichwald.^ It is true that 

 the plate is unique, so far as known, on account of the pe- 

 culiar notch in its postero-lateral borders; but an exactly 

 similar notch is to be observed in the antero-lateral borders 

 of the anterior median dorsal plate in a few specimens of 

 Bothriolepis canaäensis in the British Museum (e. g., no. P. 

 ()771), although most specimens of this well-preserved species 

 exhibit no such feature. The notched condition of the new 



1 A. S. WooDWARD, »Catal. Foss. Fishes Brit. Mus.» pt. II (1891), p. 2;]2. 



2 Traquair, Ann. Mag. Nat. Hist. [6] voL II (1888), p. 492, and ^-Mouogr. 

 Fishes O. R. Sandst. Britain-, pt. II. no. 1 (Palseont. Soc. 1894); A. S. WoOD- 

 WARD, op. cit. pt. II (1891), p. 203. 



2 



