HAMILTON CEMENT ROCK. 397 



Hhynchodus excavatus. 



Tootli small, when entire, perhaps two and a half inches long by 

 one and a quarter inches deep; crown alone preserved. Of this, the 

 external surface is marked vertically with vermicular furrows; supe- 

 rior margin sinuous, terminating anteriorly in a prominent point; the 

 superior surface irregularly excavated and roughened, showing two 

 prominent points, or tubercles, one on the middle of the exterior 

 margin, the other on the inner margin, and near the anterior ex- 

 tremity. The inner surface of the tooth shows a prominent ridge, 

 running up to the anterior point. This tooth is evidently fitted for 

 trituration rather than cutting, and resembles, in its general form, R. 

 frangens of the Corniferous limestone. It is, however, much smaller 

 and thinner, and the tubercles of the upper surface are differently 

 situated from what they are in the tooth of that species. 



Locality and formation, Hamilton Group. Brown Deer, Mil- 

 waukee county, Wisconsin. 



In addition to the fish remains, there are the following inverte- 

 brates: Of Bryozoans, a Fenestella and a tuberculated Trematopora, 

 occurring in massive and frondose forms; of Corals, the cast of the 

 cup of a Cyathophyllum ; of Brachiopoda, a new species of Lingula, 

 and also one of Discina, Orthis impressa, and an undetermined 

 Ortkis, Strophodonta demissa, S. perplana. Chonetes coron-ata and a 

 species closely resembling C. deflecta, a Productella, allied to P. 

 spinulicosta, Spirifera mucronata, S. medialis, S. granulifera, S. 

 fornacula, S. pinnata, a species allied to S. fornacula, but larger 

 and one near S. angusta, Spiriferina zigzag r , Cyrtina Hamiltonensis, 

 Trematospira kirsuta, Atrypa reticularis, A. occidentalis, Leior- 

 hynchus, resembling L. Kelloggi\ of Lamellibranchs, a Pteronites, 

 PalcBoneilo constricta. P. emarginata, a species allied to P. plana, 

 Modiomorpha concentrica, and an undetermined species; of Pter- 

 opods, an Ecculiomphalus, closely resembling Euomphalus laxus; of 

 Cephalopods, two new species of Gomphoceras, and an Orthoceras; 

 and of Crustaceans, Phacops Tana. 



These show a preponderance of Hamilton forms, some of which are 

 highly characteristic species, and occur in great abundance. With 

 these are mingled a number of Corniferous species, representing a 

 lower horizon, and a few Chemung forms belonging to a higher 

 series. 



Age. The foregoing fossils are entirely decisive as to the age of 

 the formation, and place it in the early portion of the Hamilton 

 period. This is entirely in harmony with its stratigraphical relations 

 and with the general geological structure of the interior of the conti- 



