A. Harher — Porphyritic Quartz in Igneous Rocks. 485 



for a pair of nasal openings at the antero-lateral angles of tlie snout. 

 Mesially, the plates taper somewhat to the symphysis, being in this 

 respect unlike those of Pterichthys ; and the free posterior border 

 is convexly arched. The greater part of the outer face of each plate 

 is feebly rugose and marked by a sharply bent sensory canal ; but 

 there is a conspicuous smooth band immediately adjoining the 

 posterior border, and this is further remarkable for exhibiting an 

 irregular series of minute sharp denticles different in aspect from 

 the bosses and points of the ordinary surface ornament. It appears, 

 indeed, as if the pair of plates in question formed the anterior 

 margin of the mouth, covered with an overlapping lip of soft tissue 

 and provided with a minute denticulation. It still remains to be 

 determined, however, in what manner a jaw of this form can have 

 worked ; and the opposing hard parts, if any, have yet to be 

 discovered. 



It may be added that internal to these jaw-plates in one specimeu 

 (P. 6762), there occurs the very thin lamina of smooth bone already 

 noted by Whiteaves {loc. cit. 1886, p. 103) ; and its straight hinder 

 margin is shown across the space at the symphysis in the drawing, 

 Fig. 2. This is evidently an internal bone, but it is difficult even 

 to hazard a suggestion as to its homologies and function ; and the 

 interpretation of the element must be deferred until the discovery 

 of still more satisfactory specimens. 



EXPLANATION OF PLATE XIIL 

 Upper Devonian Fishes from Scaumenac Bay, Province of Quebec, Canada. 



Fig. 1. Diplacanthus horridus, sp. nov. ; lateral aspect of fish, three-quarters 

 natural size. a. anal fin-spines ; c. caudal fin ; d^, do,, first and second 

 dorsal fin-spines ; i. intermediate ventral spines ; m. median pectoral 

 spine ; orb. orbit ; pet. pectoral fin-spine ; plv. pelvic fin-spine ; s. 

 styliforni bones ; x. pectoral arch. [Brit. Mus., No. P. 6766.] 



Fig. la. Scales of ditto, much enlarged. 



Fig. 2. Coccostens canadensis, sp. nov. ; impression of external aspect of head-shield, 

 two-thirds natural size. c. central plates ; l.occ. lateral occipital; m. 

 marginal; m.occ. median occipital; p. pineal; p.o. pre-orbital ; pt.o. 

 post-orbital. [Brit. Mus., No. P. 6755.] 



II. — On Porphyritic Quartz in Basic Igneous Eocks. 



By Alfred Harker, M.A., F.G.S. 



[Read before the British Association, Section C, Edinburgh Meeting.] 



ALTHOUGH the old-fashioned ideas as to the association of 

 different minerals held by Breithaupt and others have been 

 found to require much modification, there are still certain rules 

 which hold with a high degree of generality. They are, indeed, 

 merely consequences of the principle that the most important of 

 the factors which determine the mineralogical constitution of an 

 igneous rock, is the chemical composition of the magma from which 

 it is formed. Eoughly speaking, we may say that original free 

 silica and acid silicates occur cliaracteristically in acid rocks, more 

 basic silicates in basic rocks. The striking exceptions are few aud 

 of comparatively rare occurrence. Thus the iron-olivine fayalit^, 



