PYCNODONTS. 



71 



In the genus Pycnodus the teeth are more or less elongated, with 

 the crown shghtly expanded, and convex and smooth above. The dis- 

 position of these teeth on the vomerine bone is seen in figs. 1 and 2, 

 PI. 34. A central row of transversely oval grinders is bounded on each 

 side by a double alternate row of smaller circular teeth. Similarly 

 shaped teeth were arranged in three or four rows on the jaws ; but the 

 exact number and disposition of the maxillary teeth have not yet 

 been ascertained. 



The modifications of external form which characterize the teeth 

 of the different species of this genus, which have yet been discovered, 

 are exhibited in the great work on Fossil Fishes by M. Agassiz, 

 tab. 72, d, vol. ii. 



8ph(Brodus. — This genus is founded on detached teeth, which are 

 the sole remains of the species composing it that have as yet been 

 brought to Ught. These teeth are of a hemispherical form, with a 

 smooth upper surface, (PI. 33, figs. 1 and 2), and most probably were 

 distributed over the same bones as in the Pycnodus. The basis of the 

 tooth is a bone of the coarse cellular structure usual in osseous Fishes. 

 (PL 32) . The body of the tooth consists of coarse tubes, which arise in- 

 sensibly from the basis, where they have a diameter of ^^W th of an inch, 

 and proceed directly and perpendicularly to the surface of the tooth. 

 The characteristics of these tubes are, first, that they are so closely ar- 

 ranged together that only one-fourth of their own diameter intervenes 

 between them at their origins. Secondly, they present the appearance 

 of being composed of a closely-twisted bundle of smaller tubes,— an 

 appearance produced by the oblique direction and acute angle at 

 which the calcigerous tubes are continued from them into the clear 

 intervening substance. Besides these smaller tubes, the main trunks 

 begin immediately to give off short and somewhat coarse branches at 

 very acute angles ; these branches increase in number, and the trunks 

 proportionally diminish, until they have traversed two-thirds of the 

 vertical diameter of the tooth ; they then resolve themselves into fas- 

 ciculi of extremely minute twigs, which interlace together, and in many 

 places dilate into, or communicate with, numerous minute calcigerous 

 cells, and form so dense a layer as to intercept the light, excepting 



