a 
Genus Gyracanthus, Agassiz. 41 
largest of these supposed dorsal spines with the distal extre- 
mity of one of the least worn of those labelled “ pectoral,” we 
find a mutual approximation in character; and, further, if 
we compare both with an allied species, G. nobilis, 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 spmes of Gyracanthus tuberculatus, 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 tuberculatus, I ma 
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 speci- 
men 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, Ido not mean to affirm that the subject is thereby 
closed. Further investigation is necessary into the Irish 
Lower Carboniferous G. obliguus of M‘Coy*, and into two 
American species named G. compressust and G, Allenit by 
Prof. Newberry, the published figures of which do not indicate 
a want of lateral symmetry. M‘Coy gives an outline of the 
transverse section of G. obliquus from a position considerably 
proximal to the point, in which the two sides with the posterior 
area seem as symmetrical as in a Ctenacanthus. In such a 
spine it would be well to examine the extreme point. There 
is in the collection of the Geological Survey of Scotland a 
rather young spine from the Liddesdale beds, which I am 
* Paleozoic Fossils, p. 629, pl. iii. &, figs. 13, 14. 
t Pal. Ohio, vol. i. p. 330, pl. xxxvii. figs. 1, 2. 
t Jb. p. 831, pl. xxxvii. fig. 3. 
