218 
THE GEOLOGIST. 
most favourable circumstances. Other Trigonise with surfaces precisely 
similar are T. elongata, Sow., from the Cornbrash of the same locality, 
and from the Oxford Clay of Dorsetshire ; T. costata, var. lineolata, Ag., 
from the grey limestone of Scarborough ; another lunulate and lengthened 
form from the Upper Trigonia grit, Inferior Oolite near Stroud ; T. moni- 
lifera, Ag., from the Coral Eag of Weymouth; and T. marginata. Lye, 
from the Kimmeridge Claj'' of Wiltshire. This granulated surface occur- 
ring, as is now ascertained, in so many species of the costated Trigonia\ 
whose general forms and other characters are very dissimilar, renders it 
evident that the whole group of the Costatse is characterized by its pre- 
sence, although we may only expect to discover it occasionally in specimens 
derived from fine argillaceous deposits, and cleared simply by washing, or 
by using only a light brush. The other sections of Trigonia having tuber- 
cles, varices, or serrated ribs upon the sides, appear to have been destitute 
of this granulated tegument, as are also the recent Trigonise. I would also 
venture to remark that the value of the granulated tegument as a ground 
of distinction in the groups of testacea, does not appear to be sufficiently 
appreciated by some pala3ontologists ; that it is of higher value to us than 
as a separation between species, may be inferred from the fact that in the 
great family of the fossil Anatinidoe it characterizes all the species of the 
genera in which it occurs, and that the present appears to be the first 
known instance in which a well-defined genus can be separated into two 
sections, the one having the surface granulated, the other smooth : in Tri- 
gonia, however, it is found to pervade only a single but large and well-de- 
fined group, which in its general characters is as clearly separated from 
the other fossil groups as from the recent members of the genus. 
John Ltcett, M.D. 
Scarborough, May ^ih, 1864. 
On tlie Nehular Theory. 
Salford, Mag 17, 18G4. 
Sir, — In No. 75 of your * Geologist,' I find an article on Planetary 
Orbits, etc., written with a considerable degree of ingenuit}'-, in which you 
ask, and I suppose with no objection to a reply, for instances " of the evo- 
lution of light and heat by sloio condensation of gaseous matter." Chemistry 
supplies us with abundance of proof in this respect. One of the most 
familiar is shale loaded with iron pyrites, which, when exposed to the influ- 
ence of the atmosphere, often takes fire from the slow absorption of gaseous 
matter. 
So, on the other hand, excessive heat has greater power than chemical 
affinity, and will, if supplied in sufficient intensity, release the condensed 
oxygen again from its compound. Metals, too, have a very great power 
to condense gaseous matter within their pores, and this power is generally 
proportionate to their spongy and divided character ; but if heated, their 
affinity for gaseous bodies is likewise proportionately increased. But as I 
have just stated, heat has a greater power than chemical affinity, and there- 
fore no condensation of gaseous matter could take place until it was suffi- 
ciently cooled to be within the range of chemical power. 
Now the nebular theory aspumes that condensation is the result of slow 
cooling, and could not have taken place in any other manner ; consequently, 
no universal conflagration and condensation could simultaneously have 
taken place, as your reasoning supposes. Again, if we closely examine the 
crust of the earth, especially amongst the igneous rocks, with which we 
