248 
THE GEOLOGIST. 
The relations to Limulus are at once suggested by the form and expres- 
sion of these carapaces, while the large prominent eye-tubercles hold re- 
latively the same position as the snTall approximate oculiform tubercles or 
spots on the anterior part of the shield in Limulus (and also in Eury- 
pterus). The carapace is proportionally flatter than in Limulus, and has, 
like that, a strong thickened border ; the posterior angles rounded. The 
margin is impressed or sinuate in front, and there are slight indications of 
longitudinal grooves on each side of the central, leaving a median lobe 
proportionally wider than in Limulus. The eyes, though imperfect, re- 
mind one somewhat of the eyes of Trilobites, and are remarkably pro- 
minent. 
There is a single fragment of what appears to have been an articulation 
of the thorax, or a portion of some appendage analogous to the branchial 
feet of Limulus ; it has a flattened, curving, pointed extremity. Another 
fragment I infer may have been the caudal extremity, it is comparatively 
thick and strong ; but the specimen is too imperfect to be determined. 
The first specimen I obtained is a straight spine-like body, and I infer that 
the animal may have been provided with a caudal spine, as in Limulus. 
Such, in general, are the characters of this crustacean. Whether this 
may have been the animal which made the peculiar tracks in the sand- 
stone, I cannot say, but I have so inferred. The first specimen was found 
at a distance of thirty miles or more in a north-westerly direction from the 
locality of the tracks of Black Eiver, and in higher beds of the sandstone. 
The last found specimens are from a more distant locality, in a south- 
easterly direction, and also from beds above those of the tracks. All this, 
however, cannot furnish matter for argument against the origin oT the 
tracks, in the present state of our knowledge of a country which has been 
comparatively but little explored. 
Whatever may be proved hereafter in this respect, it does not diminish 
the great interest attaching to so new and remarkable a form of crustaceaa 
from the unequivocal primordial zone of the north-west. 
COERESPOXDENCE. 
Causes of Cosmical Changes of Temperature on our Planet. 
It is the essential character of space to be infinite. It can have no end. 
The negation of limit becomes the transcendental affirmative of the infi- 
nite. Not so of matter ; matter is finite, its position in space being regulated 
and determined by universal resultants, the effects of its mass and attrac- 
tions. Heat can only exist in that portion of space which is occupied by 
matter ; for whether caloric be a fluid or a wave, it can have no existence 
in pure space where no matter exists, pure space being a negation of every- 
thing that is material. It necessarily follows that the thermal portions of 
space must be material. 
Mr. Mackie's supposition, in the last number of the ' Geologist,' that the 
cosmical temperature of space should be the real zero, or absence of all 
heat, is correct, inasmuch as the absence of matter implies the necessary 
absence of heat ; in the chemical sense, however, it is not a zero, but a 
mere negation ; the real zero as a material condition can never be ascer- 
tained. It has been asked " on what grounds the idea of hot and cold re- 
gions of space can be maintained," etc., for I brought forward this theory 
as a solution of the glacial periods in the temperate latitudes of our earth. 
