The Fossil Elasmobranch Fishes. Sif Il 
demarcation between the Sharks with lateral gill-clefts and the 
Rays with ventral gill-clefts, seeing that there are many intermediate 
gradations of skeletal structure, of habit, and of external form by 
which one group passes into the other. Some of these gradations 
are enumerated in the following passage :—‘“ The Squatinide and 
Pristiophoride, for example, possess lateral gill-clefts, like Sharks ; 
but the structure of the vertebree, the partial growth forwards of the 
pectoral propterygium in Squatina, and several striking resemblances 
existing between Pristiophorus and Pristis and Rhinobatus, all point 
to the Squatinidss and Pristiophoridz as probably survivors of an- 
cestral Rays.” 
For the grouping of families and genera, “the arrangement 
formulated by Dr. Giinther in his Catalogue of 1870 for the recent 
forms,” is adopted, and where possible “the “extinct families and 
genera will be incorporated among them.” 
We have referred at some length to this subject of classification, 
it being an important element in the work. Although Bonaparte’s 
classification and terminology for the whole group is adopted, 
Hasse’s subdivisions founded upon the structure of the axial 
skeleton, and Cope’s new order based upon other structural 
characters, have been added; but each, as already stated, with modi- 
fications and restrictions, the result being an emphatically new classifi- 
cation, which appears to be, approximately, a good natural one, in our 
present knowledge of the fossil groups. The author modestly 
observes that “this Catalogue, however, can only be regarded as a 
provisional attempt to systematize and arrange the ascertained facts 
of Hlasmobranch Paleontology for convenience of reference.” 
In the Catalogue a diagnosis of the principal distinguishing 
characters is attached to each order, family, genus and_ species 
respectively, with comments. The nature of the type specimen, 
where preserved, if known, and the formations and localities whence 
the various specimens have been derived, are also given. The 
synonyms of the genera and species, if any, also lists, with references, 
to all the species of each genus described, as far as ascertainable, 
but of which no specimens are in the Museum, are included. All 
these details have necessitated much labour and research, the result 
being a most valuable work of special reference. 
The first Order, ichthyotomi, as already mentioned, is limited to 
those fishes having a long segmented axis in the pectoral fins, 
analogous to that in the “Ganoid Crossopterygians; it comprises 
the families Prevracanrutpm and Cuapopontipx. To the former are 
assigned the genera Pleuracanthus, Diplodus, and Chondrenchelys. 
We have here, on the very threshold of the Catalogue, interesting 
evidence of the progress of recent paleontological investigations into 
the nature and relationships of various imperfect fragments described 
by previous authors, in the merging of many genera into one. 
Thus to the genus Pleuracanthus, founded by Agassiz for the re- 
ception of some barbed spines from the Coal Measures, are referred 
no less than thirteen genera, namely, Diplodus, Orthacanthus, Xena- 
canthus, Triodus, Compsacanthus, Dittodus, Aganodus, Ochlodus, Pter- 
