TRANSACTIONS OF SECTION D. 773 



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 

 lacuna) in their bone structure, and by having the eyes in the middle of the head- 

 shield instead of at the sides. Cephalaspis, which occurs from the Upper Silurian 

 to the top of the Devonian, is the best known representative of this division. In- 

 stead 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 poly- 

 gonal 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 strougly reminds us of the lateral flaps of the CoelolepidEe. 

 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 Pleterostraci and Osteostraci in one sub-class of Ostra- 

 codermi has been strongly protested against by Professor Lankester and Dr. 0. M. 

 Reis, 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. 



Here we have a creature whose general form reminds us strongly of T/ielodus, 

 but whose close affinity to Cephalaspis is absolutely plain, were it only on account 

 of the indications of orbits on the top of the head. 



The expanded anterior part which here represents the head-shield of Cepha- 

 laspis shows not the slightest trace of cornua, but forms posteriorly a gently 

 rounded lobe on each side, clearly suggesting that the cornual flaps of 

 Cephalaspis are homologous with and derivable from the lateral expanses in the 

 Coelolepidae. This cephalic covering is composed of numerous small polygonal 

 plates like those of which the head-shield in Cephalaspis no doubt originally con- 

 sisted, and the minute tubercles which cover their outer surfaces also suggest that 

 the superficial layer was formed by the fusion of Ccelolepid scales. The body is 

 covered with rhombic scales, sculptured externally with tubercles and wavy trans- 

 verse ridges, and arranged in lines having the same general direction as the scutes 

 of Cephalaspis, from which we may infer that the latter originated from the fusion 

 of scales of similar form. The fins are as in Cephalaspis, there hemg one small 

 dorsal situated far back, and a heterocercal caudal, which is triangular in shape, and 

 not deeply cleft into upper and lower lobes as in the Coelolepidse. Finally, the 

 pcales, on microscopic examination, show well-developed bone lacunae in their 

 internal structure. 



That Ateleaspis belongs to the Osteostraci there is thus not the smallest doubt, 

 but its general resemblance to the Coelolepidas in its contour anteriorly led me 

 to regard it as an annectent form, and consequently to believe that there is after 

 all a genuine genetic connection between the Heterostraci and the Osteostraci. 

 And I have not seen reason to depart from that opinion even though Ateleaspis 

 turns out to be still closer to Cephalaspis than was apparent in the original 

 specimens. 



If this be so, then Cephalaspis, as well as Pteraspis and its allies, is traceable 

 to the Ccelolepida), shark-like creatures in which, as we have already seen, the 

 dermal covering consists of small shagreen-like scales, or of minute hollow spines, 

 and consequently all theories as to the arthropod origin of the Ostracodermi, so 

 far as they are founded on the external configuration of the carapace in the more 



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

 Upper Silurian family of Birkeniidte, but as these throw pp light as yet on the problem 

 of Pescent they may at present be only mentioned. 



