48 Bulletin 6 48 



PART IV. 



Summary. 



The detailed lists of the preceding pages show that we have 

 represented here four faunas. The work of Dr. H. S. Williams* 

 has left little to be done in determining their composition and 

 order of sequence. The efforts of the writer have therefore been 

 direcfted toward ascertaining the extreme limits of the vertical 

 range of the several species beyond their zone of culmination, by 

 a minute study of several sedlions. A precise knowledge of the 

 vertical range of the dominant species of a fauna is very essen- 

 tial to a corredt interpretation of its history. If the principal 

 species of a fauna can be shown to be entirely absent from the 

 beds below it, then it may be considered a migratory fauna. 

 The scarcity of the principal representatives of a fauna below 

 their horizon of culmination might give a locally developed fau- 

 na the appearance of having migrated into a region. 



The principal result of this study has been to extend the 

 vertical range of some of the well known species of these fau- 

 nas, and to determine more definitely that of others. The range 

 of a number of these is shown by the table. By reference to the 

 same, it will be seen that two of the most abundant and char- 

 adleristic Portage species, Glyptocardia speciosa and Lunulicar- 

 dium fragile, have been found in the midst of the Ithaca group. 

 One of the most interesting of such forms here — Spirifer Icevis — 

 has been found no feet below the well known zone at the base 

 of Ithaca falls. Some of the species of the Ithaca fauna not 

 previously known below it, have been found in the Portage 

 rocks. One of the most interesting of these is Ryhncho7iella 

 pugnus Martin, which I have found at station 7-5. Plumulina 

 plumaria has been found at a few localities associated with 

 Spirifer Icevis near the middle of the Portage. 



The number of recurrent Hamilton fossils previously known 

 from the Ithaca group has been increased by the discovery of 

 some additional species. These are Phacops rana, which occurs 

 abundantly in a single layer in the Ithaca group (station 8-4), 

 Orthis vamixemi, also abundant at a single locality (station 6-1), 

 Modio7norpha mytiloides, Niiculites triqiieter^ Strophodo7ita perpla7ia, 

 Phthonia cylindrica. 



*Bull. U. S. Geol. Surv., No. 3. 



