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have escaped decomposition, this method assumes an especial im- 
portance in fossil Ichthyology, as affording exact characteristics 
of animals long swept from the surface of the earth, and whose very 
bones have been obliterated from among the fossil witnesses of the 
early conditions of life upon our planet. By this microscopic test 
applied to the family of Sharks, Mr. Owen has confirmed the views 
of Agassiz respecting the affinities between the living Cestracion and 
the extinct genera Acrodus, Ptychodus, Psammodus, Hybodus, Co- 
chliodus ; in the case of animals also of the higher orders, he has 
settled the much-disputed places of several extinct gigantic Mam- 
malia by the same unerring test. Thus he has shown the supposed 
reptile Basilosaurus to be a Cetaceous mammifer, allied to the Du- 
gong ; the Megatherium to be, as Cuvier had considered it, more 
nearly allied to the Sloth than to the Armadillo; and the Sauro- 
cephalus to be, as Agassiz had supposed it, an osseous fish. 
Dr. Malcolmson, in amemoir on the Old Red Sandstone of the north 
of Scotland, has done important service in showing that the rocks 
composing that group are divided into three formations, the two 
lower of which are clearly distinguished from each other by their 
fossil fishes. The cornstone or central formation is charged with 
numerous remains of Ichthyolites, including Holoptychus nobilis- 
simus, a new species of Cephalaspis, and other forms not yet de- 
scribed. The lower division, consisting in this region of conglome- 
rates, shales and sandstone, is characterized by the genera Dipterus, 
Diplopterus, Cheiracanthus, &c., of Agassiz, as well as by the 
oceurrence of a singular Ichthyolite, which seems to offer close 
analogies to certain forms of Crustacea. By help of these Ich- 
thyolites, the author has been enabled to connect certain strata of 
Orkney and Caithness, and determine their relations to the beds of 
Old Red Sandstone containing fossil fishes in the basin of the Tay, 
and in the border counties of England and Wales, where they had 
been described by Mr. Murchison. 
Mr. Williamson, in a notice on the fossil fishes of the coal-fields 
of York and Lancaster, says that these coal measures are very rich 
in Ichthyolites, which abound so much at Middleton colliery, near 
Leeds, that the workmen have given to one bed the name of fish 
coal ; they are usually in fine bituminous shale above and below the 
coal, and most frequent in the roof immediately above it, where, as 
