208 GENERAL GEOLOGY. 



presence of a considerable quantity of phosphoric acid, the 

 phosphorus, without doubt, having been originally derived 

 from the decomposed bodies of the fishes. Irregular, globu- 

 lar or elongated masses of small size are not unfrequently 

 met with in the strata that contain more or less fish remains. 

 These also show a considerable proportion of phosphoric acid 

 upon analysis, and are without doubt coprolites — the 

 fossilized excrement of fishes. Besides their chemical compo- 

 sition and other characters which suggest such an origin for 

 those bodies, the microscope, and often even the unassisted 

 eye, discovers them to be partly composed of the commi- 

 nuted shells of mollusks that formed the food of these 

 rapacious tyrants of the ancient seas. 



The fish-stratum at Buffington creek can be distinctly traced 

 for the distance of half a mile, with only slight interruptions 

 by concealment, and is as well marked where it is finally lost 

 from view in both directions as it is in any part of the 

 exposure. 



In the valley of a small creek a few miles west of Burling- 

 ton, another similar stratum is to be seen enclosed between 

 firm layers of Burlington limestone. Still others have been 

 found in the same formation at Monmouth; on Honey creek 

 in Henderson county, and at Quincy, all in the State of 

 Illinois; but the most remarkable locality of the kind yet 

 discovered, is the one in Louisa county, Iowa, which has just 

 been described. 



Besides many specimens not yet studied and identified, 

 some of which are probably yet new to science, the following 

 genera have been recognized as occurring in the Burlington 

 limestone, namely: Cladodus, Petalorliynclius, Polyrliizodus, 

 Antliodus, Chomatodus, Orodus, Uelodus, Psammodus (?) 

 Cocliliodus, Deltodus, Sandalodus, Trigonodus, (and undeter- 

 mined species). These are all Selachian genera; the Graniod 

 remains are comparatively rare, and have thus far been very 

 little studied. 



In the lower division of Burlington limestone, fish remains 

 are less common than they are in the upper division, but more 



