ISTRODtTCIIOX. 
xxni 
zontally-expanded root, while all later forms are characterized by a 
deeper and more or less bifurcated base of attachment. 
With regard to the disposition of the teeth in the mouth as a 
whole, the modern Rays, most ScyUiida;, and Chlamydoselache may 
be looked upon as retaining the most primitive arrangement. In 
the predaceous Sharks there has been a tendency towards the rela- 
tive enlargement of the prehensile teeth upon the symphysis ; in 
the C««trnci(»i-like Sharks the symj)hysial teeth have become small, 
though prehensile, and the lateral teeth well adapted for tritu- 
ration. The former arrangement is particularly characteristic of 
modern times ; the latter, it is interesting to note, attained its 
maximum of specialization so long ago as the Carboniferous period. 
In many early Carboniferous genera the series of lateral crushing- 
teeth began in part to fuse into continuous plates (Pleuropla.v) ; 
two of these plates often amalgamated {Poscilodus) ; and in the 
most specialized of these “ Coohliodonts ” (e. g., Deltoptychlus) all 
traces of the boundaries of the original components of the dental 
plates became obliterated. 
Taxonomic Deductions from the Comparison op Extinct 
WITH Recent Elasmobranchs. 
In discussing the bearing of the foregoing facts upon published 
schemes of classification of the Elasmobranchii, the first point to bo 
considered is the validity of Prof. Cope’s division of the subclass 
into the two orders Ichthyotomi and SsLACHn. If the characters 
of the dentition are of any systematic importance — and when genera 
of equivalent age are under comparison wo believe they are — there 
can be no hesitation in associating the European later Palseozoic 
Pleuracanths with the skuUs of the so-called Didymodus, Cope, 
from the Permian of Texas. It is thus possible, from the researches 
of Kner, Anton Fritsch, Traquair, and C. Brongniart, to take into 
consideration all the more prominent skeletal features of these 
primitive Elasmobranchs ; and the study of nearly complete indi- 
viduals from the Middle Coal-Measures of Commentry, France, has 
lately led jM. Brongniart to attempt the restoration given in the 
accompanying fig. B *. 
As already remarked, we are inclined to believe, with Carman, 
that Cope’s determination of cartilage-bones and membrane-bones in 
the skull of “ Didymodus ” was founded upon misconception ; and 
' Op. cil. p. 7, fig. 2. 
