370 Reviews—British Museum Catalogues— 
sorts of fishes, each with the root unto it, to make it the very mark- 
able, and right proportion of such a kind of tongue in all respects.” 
Their true nature was first made known by Steno in 1660, and 
later by Scilla in 1752, who compared them with teeth of recent 
Sharks. The bony spines were supposed to be jaws, and some even 
to be plants. That they were the dorsal fin-spines of extinct Hlas- 
mobranch fishes was first proved by Buckland and De la Beche 
about 1850, who proposed the term ‘Ichthyodorulites.” <‘ Agassiz 
named many of them, and assigned a few to their correct zoological 
position.” Agassiz is also credited as being “ the first to place the 
study of Hlasmobranch Paleeontology upon a truly scientific basis ; ” 
and his great work as still forming “the groundwork of the whole 
subject.” The researches of numerous subsequent authors on the 
same subject, whether of a general character or limited to a descrip- 
tion of the fauna of a geological horizon, of a country, or a district, 
are respectively noticed. 
In a ‘Synopsis of Paleontological Results,” the author states 
that the general results of these discoveries and investigations add 
much that is new, and the main points of biological significance 
are briefly enumerated. 
To give an intelligible résumé of the descriptions of the internal 
and external skeleton and the dentition would exceed the limits 
of this notice. The respective portions described are first the 
“ Cartilage”; and this, even in the Lower Carboniferous, “exhibits 
a considerable amount of calcification.” The ‘“ Head and Visceral 
Arches” come next, and all that is known regarding the structure 
of the extinct Elasmobranch skull in the various genera is fully 
detailed. The structure of the “ Vertebral Column,” and its import- 
ance as an element in the classification of the group, has been 
already referred to. The structural characters of the “‘ Pectoral and 
Pelvic Arches and Fins,” respectively, and of the ‘‘ Median Fins,” 
follow; the “ Shagreen and Dermal Defences” are the next objects 
of comment, and finally the “ Dentition.” 
With respect to the adoption of a classification for the extinct 
Sharks and Rays, the author observes, that “the first point to be 
considered is the validity of Prof. Cope’s division of the subclass 
into the two orders IcurHyotom1 and Sexacui.” The first order 
was founded upon certain modifications observed in the structure 
of the skull, the pectoral fins, pelvic arch, and the axial skeleton of 
the Huropean Pleuracanthus and the American Didymodus; these 
modifications being sufficiently pronounced as to justify an ordinal 
separation from the other groups of the subclass. But the principal 
character is the structure of the pectoral fin, in which the metaptery- 
gium forms a long segmented central axis, and as this structure 
‘differentiates the Crossopterygii from the higher Ganoidei or 
Actinopterygii,” the same character separates the Ichthyotomi— 
though perhaps less widely—from the Selachii. The order there- 
fore, as here defined, is limited to those fishes that possess this type 
of fin. The reason for the inclusion of the Sharks and Rays in one 
order is explained by the difficulty of defining a distinct line of 
