()i) Tl><' American Geologist. Jan. i89i 
One need only take a glance at the dentition of the Port Jackson shark, 
to see how readily such double naming may occur. This remark is yet 
more pertinent in the ease of fossil botany. 
Thus Mr. Lydekker remarks: "it is probable that some of the dor- 
sal fin-Spines, originally described under the name of Ctenacanthus be- 
long to Orodus (more properly Oreodus). Most of these spines are how- 
over referable to the allied Carboniferous genus Sphenacanthus. " 
In regard to the remarkable fossil Edestus the author accepts Dr. 
Newberry's opinion that "we may regard them as the dorsal spines of 
large cartilaginous tishes of which the other parts are as yet unknown 
and may suppose that they were used for attack and defence as the 
spines of Trygon or of Acanthurus. '* 
Fossil chimaeroids are comparatively few and chiefly Mesozoic or later 
in date. Rhynchodus and Ptyctodus being the most important forms of 
reputedly palaeozoic age. 
On the other hand the Dipnoans indicate an essentially ancient ori- 
gin by their wide distribution over the world of their few remaining 
representatives. 
The great, but difficult order of ganoids, as the term is here employed, 
includes nearly all the oldest tishes such as Cephalaspis and Pteraspis 
and those Old Red sandstone fossils with which the writings of Hugh 
Miller have made us all familiar. 
In this connection we may remark that we believe that the sensory 
system has not been found in the shield of the Cephalaspidce, though it 
is now known to have existed in the shield of the Pteraspldce. There is 
also some error regarding the locality of Holaspis but Lankester's paper 
is not at hand to supply the correction. No Pteraspidian except Scaph- 
aspts is known from undoubted Silurian rocks in England so that 
Auchenaspis from the passage-beds of Ledbury, and Didynuispls of which 
only a single specimen is known, should be referred to the Devonian. 
Scaphaspte is now by many considered the ventral shield of Pteraspis, 
but in this case it is not easy to understand why it alone should be 
found in the Lower Ludlow. Only a very few specimens are known, 
but possibly more than one species is included in them. The Dlplaspis 
of Matthew should have been here mentioned, from the Silurian strata of 
New Brunswick containing a marine fauna resembling that of the 
English Ludlow. According to its author's description this species had 
a ventral as well as a dorsal shield. 
Here also belong those gigantic fossils, which have been recently 
brought to light from the Black Shale of Ohio, such as Dinichthys and* 
the yet huger Titanichthys. The order is distributed by the author into 
twenty-six families. It is needless to add that he adopts in this de- 
partment, the recent work of Mr. A. S. Woodward and Dr. Traquair 
for Great Britain and of Dr. Newberry for North America. 
The teleosts are. says our author, in all probability descended from 
the ganoids and occupy in the class a somewhat analogous position to 
that held by the Squamata among the reptiles and the Passeres among 
