96 Proceedings of the Boijal Physical Society. 



mity of one of tlie least worn of those labelled " pectoral/' we 

 find a mutual approximation in character; and, further, if 

 we compare both with an allied species, G. nohilis (Traq.), 

 from the Edinburgh district, pretty large specimens of which 

 sometimes occur with the points very slightly worn indeed, 

 the whole matter is cleared up. I have now no longer any 

 doubt that the spines of Gyracanthus tuherculatus, sup- 

 posed by Messrs Hancock and Atthey to be "dorsal," are 

 simply young specimens of the very same spines classed by 

 them as "pectoral," and represent the distal portions or 

 extremities, which in the adult spines have been lost by 

 attrition. These spines increased by progressive growth at 

 the base, and as they grew, progressive differences in sculpture, 

 amount of lateral compression, and so on manifested them- 

 selves ; so that the young spine is not a miniature of the old 

 one, but represents only a distally situated portion of it, 

 greater or less as the case may be. And in the case of the 

 Newsham specimens of Gyracanthus tiiberculatus, I may 

 mention, as a final and convincing proof, that, although 

 Messrs Hancock and Atthey state that in the spines supposed 

 by them to be dorsal the pointed extremities ''are all perfect, 

 not being in the least worn," I find in one so labelled, a 

 specimen 11 inches in length, very distinct wearing already 

 in progress just in front of the tip. 



Although Messrs Hancock and Atthey's dorsal spines of 

 Gyracanthus are certainly not so, and although, since my 

 attention was directed to the subject, I have not been able to 

 find in any collection, public or private, spines of this genus 

 to which I could assign a median position, and am conse- 

 quently inclined to doubt the presence of dorsal spines alto- 

 gether, I do not mean to affirm that the subject is thereby 

 closed. Further investigation is necessary into the Irish 

 Lower Carboniferous G. oUiquus of M'Coy,^ and into two 

 American species named G. compressus^ and G. Allcni^ by 

 Prof Newberry, the published figures of which do not indicate 



1 Paleozoic Fossils, p. 629, pi. iii. k, figs. 13, 14. 



2 Pal. Ohio, vol. i., p. 330, pi. xxxvii., figs. 1, 2. 



3 Thid., p. 331, pi. xxxvii., fig. 3. 



