OS 



THE GEOLOGIST. 



have been saved, and are now in my possession. Although the matrix in 

 which they have been preserved is so very coarse that the various layers 

 composing the cephalic shield are not sufficiently preserved to show their 

 characteristic structure, and hence Professor Huxley failed to identify my 

 specimens with any of the named English species, yet its form is very per- 

 fectly exhibited ; and in the cast, the spine which proceeded from the 

 posterior part of the head is well shown. The rough sketch I herewith give 

 is a tolerably correct representation, and is of 

 the size of nature ; from this it will be seen 

 that it much more nearly coincides with Pro- 

 fessor Huxley's restoration than with that of 

 Mr. Mitchell. The head-plate had evidently 

 been formed of two parts, the anterior resem- 

 bling in shape the head of a small Cephalaspis, 

 but rather more elongated ; the posterior por- 

 tion is by much the larger of the two, its 

 length being nearly two and a half times 

 that of the anterior ; its shape is nearly oval, 

 truncated behind, with a short cusp or horn 

 on each side, and in the centre stretching 

 backwards and upwards, terminating in a 

 sharply pointed spike or spine. A very dis- 

 tinctly incurved ridge, but of no great eleva- 

 tion, commences about halfway back on the 

 posterior plate, and terminates in the above- 

 mentioned spine. There are indications at 

 the edges of the head, very close to the junc- 

 tion of the two plates, of what may have been 

 the eye orbits, but these are indistinct. The 

 principal if not the only points of divergence 

 betwixt the specimen and Professor Huxley's 

 restoration are, the position of the lateral ter- 

 minal cusps, and the absence of any division 

 betwixt these and the head-plate, and in the 

 form and size of the posterior elongation (nu- 

 chal spine), which in my specimen seems to be a well-formed, round, 

 sharply pointed spine. 

 ^ Mr. Lankester, in comparing Mr. Mitchell's restoration with that of 

 Professor Huxley, does not, in my opinion, sufficiently allow for what may 

 have been specific differences of form ; but at the same time, unless Mr. 

 M itchell Bads what he figures as the prolonged central termination, in such a 

 position as to afford undoubted evidence of its forming part of the same 

 bead, its size seems to me so sadly exaggerated that I cannot but regard it 

 aa having formed part of some other, probably very different creature. 

 What renders this the more likely is, that I have examined many frag- 

 ments vt hu h. although too imperfect to found ou for any new genus, seem 

 to belong to sonu> nearly allied form, and are evidently parts of neither 

 Cephalaspis nor Ptera&pis. I am also much inclined to suspect that Mr. 

 Mitchell* third figure has been built up of such fragments. Several 

 beads oi Cephalaspis which have been in my hands, go far to show that 

 the larger part ot the under portion of this fish's head had been covered 

 by internment : and judging from the many points in which Cephalaspis 

 resembles the Sturgeon of our own seas, I have little doubt but that, like 

 u. it was Furnished with a sucking apparatus for its mouth: and in all 

 probability the under portion of the head of Pteraspis was similarly 



