F. R. C. Reed—On the genus Trinucleus. 169 
IVY. Concuusron. 
Detrital mineral assemblages are of considerable value for strati- 
graphical purposes over limited areas, but except for very broad 
divisions, as e.g. Hocene and Pliocene beds, cannot be used for wide 
correlations. Unlike fossils, minerals cannot show an evolutionary 
sequence or gradual variation, and there being comparatively so few 
species occurring in detrital sediments the number of combinations is 
very limited. Mineral suites, unlike life assemblages, therefore tend 
to recur. As in the case of minerals of igneous rocks, the relative 
abundance of the heavy components in sediments is of great determi- 
native value. Mere lists of detrital minerals occurring in sediments 
are therefore not sufficient; we need to know the size, colour, form, 
degree of alteration (if any), and other characters of each mineral, as 
well as its relative abundance. 
The zonal value of mechanical and mineral analyses has not yet 
been determined, partly because the rocks which yield the best 
heavy mineral crops, such as sandstones, loams, ete., do not lend 
themselves to zoning by fossils, while the clays and limestones, 
which often contain valuable life assemblages, contain frequently 
only a very small quantity of heavy minerals (other than authigenic). 
The connexion between the mechanical composition (e.g. of clays), 
as indicating differences in condition of deposit, and the contained 
faunas of various zones, has yet to be worked out, but the results 
may well be interesting. 
The goal to be sinned at is a Ienewiedies of the mineral and 
mechanical composition of every sedimentary rock in the British 
geological column, collection of material being made over as wide 
an area as possible. Cores of borings, as well as the records only, 
must be carefully preserved, and the beds met with subjected to 
similar analyses. The information obtained should be correlated 
with that yielded by the distribution of isopachytes and sub-surface 
‘contours. 
V.—Sepewick Musrum Norss. 
NorEs on tHE GENUS ZrInucLEUS. Part IV. 
By F. R. COWPER REED, M.A., Sc.D., F.G.S. 
(Concluded from the March Number, p. 123.) 
MorpHoLocicaL CoRRELATIONS. 
EK have now finished the survey of the characters of the head- 
shield of Trinucleus sufficiently to enable us to discuss the 
vexed questions of the affinities, systematic position, and evolution of 
the genus; and from the foregoing details of the various species we 
can perceive that there are many difficult problems presented when 
we attempt to correlate its structure with that of other trilobites. 
Whether the peculiar characters of the genus are the result of 
degeneration or of the persistent retention of early phylogenetic 
stages or of a reversion to such stages is a matter of dispute. 
