GEOLOGICAL SURVEY IN SILURIAN ROCKS OF SOUTH OF SCOTLAND. 857 



tion, and would include under them the following families, in their order of special- 

 isation : — Ccelolepidce, Psammosteidce, Drepanaspidce, and Pteraspidce. 



Heterostraci and Osteostraci. 



Although the classification of the Orders Heterostraci (Pteraspids), Osteostraci 

 (Cephalaspids), and Antiarcha (Asterolepids), together in one great division or class 

 of " Ostracodermi," as adopted by Cope and Smith Woodward, is, in the present 

 state of knowledge, very convenient, and has for that reason been used by myself, 

 it has not, however, gained universal assent. Reis protests against the union of the 

 Pteraspids and Cephalaspids as " unbegrtindet " and " unheilvoll," while Prof. 

 Lankester, who thirty years ago treated them in his classical monograph as " sections" 

 of one group, # has emphasised another view of the matter in the short paper in 

 Natural Science to which I have already referred (xv. p. 46), and where he says — 

 " There is absolutely no reason for regarding Cephalaspis as allied to Pteraspis beyond 

 that the two genera occur in the same rocks, and still less for concluding that either 

 has any connection with Pterichthys." 



I must nevertheless hold that the configuration and structure of the remarkable, 

 though imperfectly known genus Ateleaspis does seem to indicate that there is, after 

 all, an actual connection between the two groups. So far as the external form of 

 Ateleaspis, shown in the specimen represented in PI. IV. fig. 6, is concerned, the 

 resemblance to Thelodus is so striking, that the idea of a genetic connection between 

 them is well-nigh unavoidable, and in truth I placed it at first immediately after the 

 Ccelolepidae in the order Heterostraci, even although certain misgivings were aroused in 

 my mind by the crescentic markings (see text fig. 2) which seemed to point to the 

 presence of orbits on the top of the head as in Cephalaspis. But when I succeeded in 

 obtaining microscopic sections of the scales, and saw that the indications of a Cepha- 

 laspis-like position of the orbits were correlated with the presence of true bone lacunae 

 in the dermal hard parts, then I felt compelled to transfer Ateleaspis to the Osteostraci, 

 though with a tolerably strong conviction that we have here an annectent form — in 

 fact, a "missing link." Here, however, it must form the type of a very distinct 

 family, characterised especially by the want of the pre- and post-orbital openings or 

 markings of the Tremataspidae and Cephalaspidae, the acute lateral cornua of the latter 

 being also absent. We do not know if the shield was flexible ; at all events its com- 

 position, externally at least, out of a multitude of polygonal tuberculated plates gives 

 it a resemblance to that of Cephalaspis as well as of Psammosteus. For though the 

 apparent tessellation of the shield of Cephalaspis may be " deceptive," inasmuch as the 

 tesserae are not actually separate from each other, but take part in the formation of 



* It must, however, not be forgotten that Prof. Lankester in the concluding page of that monograph (xiv. p. 62), 

 even then stated that " The Heterostraci are associated at present with the Osteostraci because they are[found in the 

 same beds, because they have, like Cephalaspis, a large head-shield, and because there is nothing else with which to 

 associate them." 



