Notices of Memoirs — Br. R. H. Traquair's Address. 467 



rostrum in front of the mouth; and it is to be noted that the small 

 round apertures usually supposed to be orbits ai-e in a position quite 

 analogous to that of the sensory pits in Drepanaspis. The plates of 

 the carapace of Pteraspis are not, however, tuberculated, but orna- 

 mented by fine close parallel ridges, the microscopic structure of 

 which, along with their frequent lateral crenulation, leaves no doubt 

 in our minds that they have been formed by the running together in 

 lines of Thelodm-\\kQ shagreen grains. An aperture supposed to be 

 branchial is seen on the plate forming the posterior angle of the 

 carapace on each side. 



Until these recent discoveries concerning the Coelolepidee and 

 Drepanaspidfe, Pteraspis and its allies, Cyathaspis, and Palceaspis 

 constituted the only family included in the oi'der Heterostraci of 

 the sub-class Ostracodermi, distinguished, as shown by Lankester, 

 by the absence of bone lacunse in the microscopic structure of their 

 plates. It is now, however, clear that we can trace them back to an 

 ancestral family in which the external dermal armature was still in 

 the generalized form of separate shagreen grains or spinelets. 



But the Ostracodermi are usually made to include two other 

 groups or orders, namely the Osteostraci and the Asterolepida.^ 



The Osteostraci are distinguished from the Heterostraci by the 

 possession of lacunse in their bone structure, and by having the 

 eyes in the middle of the head-shield instead of at the sides. Gepha- 

 laspis, which occurs from the Upper Silurian to the top of the 

 Devonian, is the best known representative of this division. Instead 

 of a carapace, we find a large head-shield of one piece, though its 

 structure shows evidence of its having been originally composed of 

 a mosaic of small polygonal plates, and it is also to be noted that 

 the surface is ornamented by small tubercles, there frequently being 

 one larger in size in the centre of each polygonal area. The 

 posterior external angles of the shield project backwards in a right 

 and left pointed process or cornu, scarcely developed in C. Murchisoni, 

 internal to which, and also organically connected with the head- 

 shield, is a rounded flap-like structure, which strongly reminds us 

 of the lateral flaps of the Coelolepidae. The body is covered with 

 scales, which on the sides are high and narrow ; there is a small 

 dorsal fin, and the caudal, though heterocercal, is not bilobate. It 

 is scarcely necessary for me to add that we find just as little evidence 

 of jaws or of teeth as in the case of the Heterostraci. 



The association of the Heterostraci and Osteostraci in one sub- 

 class of Ostracodermi has been strongly protested against by Professor 

 Lankester and Dr. 0. M. Eeis, but here the Scottish Silurian strata 

 come to the rescue with a form which I described last year under 

 the name of Ateleaspis tessellata, and of which some more perfect 

 examples than those at my disposal at that time have recently come 

 to light through the labours of Mr. Tait, of the Geological Survey of 

 Scotland. 



1 To these I myself recently added a fourth, the Anaspida, for the remarkable 

 Upper Silurian family of Birkeniidte, but as these throw no light as yet on the 

 problem of Descent they may at present be only mentioned. 



