Trans. N. V. Ac. Set. 146 May 7, 



roof of the mouth, to those of Ceratodus, will strike any one who ex- 

 amines them ; and no other analogy in the whole range of ichthyic den- 

 tition suggests itself. 



Until we shall obtain further information in regard to this strange 

 system of dentition, it will be safest therefore to refer these fishes 

 to the group which includes Dipiertis, Paledaphus, Ctenodus and 

 Ceratodus. 



MYLOSTOMA VARIABILIS, N. 



Inferior dental plates, ovoid, or short-spatulate in outline, 3 inches 

 long by i^ wide in the widest part; crown composed of dense bony 

 tissue, becoming enamel-like near the surface, 6 lines thick anteriorly 

 but thinner toward the narrow posterior end. Surface granular or 

 roughened with a vermicular marking, raised, near the middle and 

 on the outer side, into a strong, oblique tubercle or boss. Below, the 

 anterior portion of the tooth is excavated into rudely concentric fur- 

 rows, from the centre of which descends, beginning at the anterior 

 third, the obliquely compressed splenial bone, which projects down- 

 ward and backward, to the length of several inches, becoming i % inches 

 wide at its widest part. 



Considerable diversity is shown in the degree of irregularity of the 

 crown surface in corresponding teeth. Three of these, nearly of the 

 same size, show in one a rudimentary irregular boss near the outer 

 angle ; another, the opposite tooth, rises in a strong furrowed, poster- 

 iorly depressed, obtuse tubercle, half an inch in height ; while the third, 

 corresponding in position to the last, is a little shorter and broader, and 

 the tubercle is latterly deflected and compressed, till it forms a blunt 

 edge. 



Still another and very imperfect tooth, of a smaller size, has a 

 crown elliptical in outline and a blunted, furrowed tubercle, relatively 

 much larger than either of the others. This smaller tooth varies so 

 much from the others, with which it is associated, as to lead to the 

 suspicion that the dentition of the lower jaw like the upper consisted of 

 more than one pair of dental plates. 



The dental plates of the upper jaw, in one or several pairs, are 

 rudely triangular in outline, with a flattened or concave triturating 

 surface, bearing, as do some of the inferior teeth, evidences of the 

 wear occasioned by the prominent bosses of the opposing teeth. 

 The surface of attachment of these dental plates to the cranium is 

 fiat or concave, and somewhat rough from the coarse cellular tissue 

 of the bone. The sides are straight or beveled, apparently for 

 co-adaptation, and, by their character, favor the conclusion that the 



