2 Trans. Acad. Sci. of St. Louis. 



fore, because such families as Codasteridae , Olivauidae, 

 etc. are known to occur only in such limited quantity and in 

 such a poor state of preservation that conclusions regarding 

 their internal structure will stand no comparison with that of 

 the Pentreniidae and leave a large and open field for future 

 investigation. 



The present revision and the anatomical descriptions herein 

 given are based mainly on Pentre7nites sulcatus, fiorealis and 

 conoideiis, not because they are more abundantly represented 

 than any other species (for Pentremites godoni occurs in 

 great numbers in Pulaski Co., Kentucky, as well as at Hunts- 

 ville, Alabama), but the preservation of Pentremites sulcatus 

 is so excellent that we often find the most delicate and fragile 

 organs preserved, especially in those specimens which were 

 imbedded in a kind of clayey substance. I think it is not 

 unreasonable to suppose that similar conditions in structure 

 must have existed in other species belonging to this family. 

 Therefore, if not otherwise stated, all descriptions and 

 illustrations are based on Pentremites sidcatus, fiorealis and 

 conoideus for the following reasons : — 



1st. These species are found in the most perfect state of 

 preservation . 



2nd. They are among the largest species so far discovered. 



3rd. They are among the most abundant species to be found 

 here. 



I take this opportunity to express my sincere thanks to the 

 following scientific friends for the generous and ready way in 

 which they have facilitated my undertaking by the loan of 

 valuable specimens: Prof. W. H. Barris, of Davenport, 

 Iowa; Mr. R. A. Blair, of Sedalia, Mo. ; Dr. J. H. Britts, of 

 Clinton, Mo.; Prof. G. C. Broadhead, of Columbia, Mo.; 

 Mr. G. K. Greene, of New Albany, Ind. ; Mr. Edwin G. Kirk, 

 of Burlington, Iowa; Dr. Joshua Lindahl, of Springfield, 111. ; 

 Mr. F. A. Sampson, of Sedaha, Mo.; Mr. C. Schuchert, of 

 the Smithsonian Institution, Washington, D. C. ; Prof. A. 

 G. Wetherby, of Cincinnati, Ohio; Dr. W. P. Jenney, and 

 Mess. J. and H. Hurter, of St. Louis, Mo. 



It is a well-known fact that all principal parts in the con- 



