184 PALEOZOIC FISHES OF NORTH AMERICA. 
these had been driven from the open sea, as they were not capable of suc- 
cessfully competing with the varied and powerful Sharks. 
Some other interesting things also come out in the study of the Car- 
boniferous Elasmobranch fauna. 
Judging from the teeth, which are the most characteristic organs, we 
may infer that in the Carboniferous age the Elasmobranchs as a whole were 
far less sanguinary and formidable than now. At least three-fourths of all 
the species described had crushing teeth, adapted to the trituration of sea- 
weeds or to breaking the shells of mollusks and crustaceans, and the number 
of those provided with cutting or piercing teeth was comparatively small. 
Most of the teeth were considerably like those of the Port Jackson shark, 
and there is little doubt that, as suggested by Agassiz, this peculiar mem- 
ber of our present fauna is a descendant and a representative of the most 
numerous and characteristic tribe of Carboniferous Plagiostomes. 
That the food of these fishes was generally mollusks, and sometimes 
those with very resistant shells, is proved by the massive character of the 
dentition, the pitted or ridged surface of the enamel to prevent the slipping 
of the objects crushed, and also by the evidences of wear at the places where 
the greatest mechanical effect was gained. 
The simplest form of dentition adapted to crushing was that of Fsam- 
modus, of which the teeth were quadrangular, from one to three inches 
square and half an inch to an inch in thickness, composed of solid dentine 
below and a thick sheet of pitted enamel above. Closely allied to Psam- 
modus was Archcobatis, of which the teeth are figured on the plates accom- 
panying this memoir. The dentition of this genus formed a pavement of 
many teeth, of which the largest were six inches long by four inches wide 
and one and a half inches thick. To prevent the slipping of the objects 
operated upon by this powerful crusher the enameled surface was roughened 
by transverse, parallel ridges, precisely as in the living Rhynchobatus. 
The first deviation from this simplest style of dentition is found in 
Sandalodus, Deltodus, Cochliodus, etc., in which the teeth are of different 
sizes and forms on the same jaw, and are more or less arched or twisted. 
The next is Chomatodus, in which the teeth were long and narrow, flat or 
ridged on the upper surface, and placed transversely, like those of the 
