12 



DAVENPORT ACADEMY OF NATURAL SCIENCES. 



angular, and somewhat irregular. The lateral teeth are uniformly 

 marked by a regular fringe of slender denticles, which are much 

 smaller on the outer lateral. 



"Some portions of the membrane and different specimens show con- 

 siderable variation in the length and strength of the denticles on the 

 lateral teeth, and sometimes their number seems subject to some 

 mutation. The numbers given in the formulae were averaged from 

 several enumerations, and represent the comparative denticulation of 

 the teeth." 



DESCRIPTIONS OF FIGURES. 



(All figures enlarged to 500 diameters.) 



FIG. i. Rhachidian teeth. 

 " 2. Intermediate teeth. 

 " 3. Inner laterals. 

 " 4. Outer laterals. 

 " 5. A portion of the odontophore, representing- the teeth in their natural position. 



