Dt\ Icor Thomas — JVote on P/iacops Icevis. 167 



Under the microscope the matrix is seen to consist of a coarse- 

 grained aggregate of a much twinned felspar, at least as basic as 

 labradorite. There is a little pale-green hornblende scattered through 

 it irregularly. The corundum is quite colourless, shows a very rough 

 surface, and has many irregular cracks besides the well-defined 

 parallel partings. It is traversed by occasional veins of a brightly 

 polarizing alteration product (?diaspore). Round the corundum is 

 the dark substance noticed in the hand-specimen. This proves to be 

 principally a deep-green spinel, no doubt pleonaste, in gi'anular 

 aggregates, associated with a good deal of magnetite. The grains 

 are perfectly isotropic, and have no definite shape even where in 

 contact with the felspar, but there are indications of rectangular 

 cleavage traces, and in some cases the granules are darkened by 

 close-set parallel striations like those sometimes observed in augite. 



It is difficult, without having made an examination of the locality, 

 to offer any useful suggestions regarding the origin of this interesting 

 association of minerals. 



VII. — A Note on Piiacops {Tj^imerocephalvs) l.evis (Mtjnst.). 

 By Ivor Thomas, Ph.D., B.Sc, F.G.S.' 



THE British form recognized under this designation appears to 

 have been first recorded and figured from this country by 

 J. de C. Sowerby (16, pi. Ivii, fig. 30), who named it Asaphus or 

 Trimideus ('?). He remarks " that no trace of the position of the eyes 

 remains on the cast, which is also the case with the genus Trinucleus, 

 Murchison". This form was afterwards allocated by Salter to 

 Phacops {TrimerocepTialiis) Icevis (Miinst.) (13, p. 16, pi. i, figs. 5-7; 

 14, p. 1, pi. ix, figs. 1-5). Phillips had previously named the 

 same species Calymene Icevis^ Mtinst. (8, p. 129, pi. Iv, fig. 250), 

 a determination which both Salter (13, p. 16) and M'Coy (6, p. 404) 

 showed to be at fault. This was complicated by the fact that 

 Miinster (7, pi. x, fig. 6; 7«, pi. v, fig. 4) had described two different 

 forms under the same specific name, Jrinucleus'^ l(evis\ and Calymene 

 Icevis, the latter of which Salter (13, p. 18) correctly adjudged to 

 be a synonym of Phacops granulahis (Miinst.) {Calymene auct.) 

 (7«, pi. V, fig. 3). M'Coy had previously noted the identity of C. lavis. 

 Miinst., with Calymene gramilata of the same author, but assigned it 

 to his genus Portlockia (6, p. 404). 



Miinster's figure of T.'llcBvis, which gave the specific name to 

 Ph. [Trimerocephalus) Icevis of Salter, is very poor, and a determination 

 by comparison with it would appear at present to be quite unsafe. 

 A similarity certainly exists between the two forms in the absence 

 of eyes and in the smooth character of the surface. But distinctions 

 exist; for instance, among other details, the ratio of the length of 

 the head-shield to its width is considerably less in Miinster's figure 

 than in the British form, while the broad marginal border seems 



1 Communicated by permissiou of the Director of the Geological Survey. 



