254 Br. R. H. Traquair — On Diplacanthus, sp. nov. 



intrusive rock of a given sized grain, the greater vs^ill be the amount 

 of contact metamorphism ; also his observation is a most valuable 

 one concerning the difference in the type of such change in the 

 surrounding rocks, as the composition of the acting magma varies. 



Looking at this valuable evidence so lucidly worked out by Prof. 

 Brogger in the light of no preconceived idea, we find that it does 

 not in any way prove " that differentiation of the original magma 

 has, to an essential degree, been dependent on the laws which 

 govern the sequence of crystallization," but that it does afford the 

 strongest evidence in favour of the differentiation of an original 

 unique paste by the osmotic reciprocal reaction between it and that 

 of the country rock that it has been intruded into.^ 



I am satisfied that when these rocks are studied in this new light 

 and in conjunction with other eruptives of the district, it is not 

 improbable that we shall find they are all modifications of one 

 original paste (not that suggested by Prof. Brogger) which possibly 

 might be that of the great mass of granitite of the region. 

 Naples, March 16th, 1894. 



lY. — On a New Species of Diplacanthus, with Eemakks on the 



ACANTHODIAN ShOULDER-GikDLE. 

 By R. H. Traquair, M.D., F.R.S., F.G.S. 



MANY years ago, and before I entered on my duties in the 

 Edinburgh Museum, the late Mr. C. W. Peach pointed 

 out to me a specimen in the Hugh Miller Collection, which he 

 was inclined to consider as new. It was a Cromarty nodule with 

 dislocated remains, including a dorsal and a pectoral spine, of what 

 was apparently a Diplacanthus of unusual size. 



I did not consider at the time that the evidence was quite sufficient 

 to warrant the description of this specimen under a new specific 

 name ; but since that time, and indeed comparatively recently, 

 specimens have been added to the collection which fully justify 

 Mr. Peach's surmise. Certainly they belong to an undescribed 

 species, and this species is undoubtedly the same as that in the 

 Hugh Miller Collection already referred to. Two of them were 

 purchased two years ago from Mr. Damon, while one occurred in 

 the Powrie Collection ; all three are from Gamrie. 



This species attains a greater size than D. striatns, Ag., the 

 largest entire specimen measuring 7^ inches in length, while still 

 greater dimensions are indicated by the spines of the example in 

 the Hugh Miller Collection. The great distinguishing peculiarity 

 is to be found in the external sculpture of the spines. The longi- 

 tudinal ridges are fewer and broader, the intervening furrows 

 narrower, than in D. striatus, and the flattened surfaces of these 

 ridges are again finely striated. On the posterior dorsal spine only 

 two lateral furrows are seen, by which two ridges are marked off in 

 front, the space remaining behind being nearly equal to both of these. 



1 See my paper, "The Causes of the Variation in the Composition of Igneous 

 Eocks," Nat. Science, vol. iv. Feb. 1894. 



