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TITANOTHERES OF ANCIENT "WYOMING, DAKOTA, AND NEBRASKA 



Summary. — The usage adopted in this monograph 



is as follows: 



Holotype (of original autlior) : The original individual type 

 specimen selected by the author. 



Cotypes: Different individual specimens rightly or wrongly put 

 together by the author as "types." 



Paratype (of original author) : Additional individual specimen 

 or specimens noted by the author in the original description 

 and used by him in defining the species. 



Lectotype (of subsequent author) : The specimen selected by 

 a subsequent author, from among the "cotypes," for pur- 

 poses of subsequent description or redefinition. This may 

 be (a) the specimen first mentioned by the author, or (6) the 

 specimen to which the specific name obviously applies. 



Neotype (of second or subsequent author) : A new specimen 

 selected in a subsequent description because of the loss or im- 

 perfection of the holotype or type. 



These five terms are all that are necessary in verte- 

 brate paleontology. The terms plesiotype and hypo- 

 type are discarded in this monograph because they are 

 too indefinite. 



Monographic revision in the use of above terms. — 

 Leidy founded the species Palaeosyops paludosus 

 upon some isolated teeth from the low levels of 

 Church Buttes. In the original description these 

 teeth, which probably represent more than one in- 

 dividual, were treated as coordinate or equivalent 

 types or "cotypes." Out of this lot the second 

 lower molar (m2), which was the first specimen men- 

 tioned and described by Leidy, has been selected by 

 Osborn in the present volume as the final standard, 

 or "lectotype," of the species. 



In the same original description by Leidy of P. 

 paludosus a second lot of teeth, from the high levels 

 of Henrys Fork, were mentioned, and the characters, 

 of these teeth entered into Leidy's original conception 

 of the species. These teeth are now called "paratypes." 



In the present revision, since there is little doubt 

 that Leidy's paratypes are not really conspecific with 

 the specimen first mentioned (lectotype), Osborn 

 has selected from the same geologic level. Church 

 Buttes, a lower jaw in which m2 agrees most clearly 

 with the lectotype m2 and which is to serve as a 

 secondary type, or "neotype." 



It wUl be seen that cotypes, paratypes, or neotypes 

 may sometimes be wrongly associated specifically 



with the holotype, in which case the specific name 

 must cling to the holotype and lectotype as the 

 ultimate standard means of identification. 



The first step toward permanence, therefore, is the 

 settlement of the holotype characters, which is some- 

 times an almost impossible task, owing to the poor 

 quality of the holotype selected — for example, the 

 holotype of Palaeosyops major Leidy, a jaw fragment 

 without teeth; the holotype of P. humilis Leidy, a 

 single deciduous premolar. 



Rule 5. Avoid mingling as types and cotypes specimens from 

 different geologic levels. 



The mingling of types and cotypes from different 

 geologic levels has been the second chief source of 

 confusion. To cite a prominent instance. Cope's 

 cotypes of Palaeosyops laevidens were two skulls col- 

 lected at widely separated localities, and in his original 

 description no regard was shown for their possible 

 difl'erence of geologic age. It appears almost certain 

 that the lectotype belongs to a lower level and is 

 perhaps some thousands of years more ancient than 

 the paratype. Similarly we have shown that the 

 lectotype of Leidy's P. paludosus is from Bridger 

 level B 1 or B 2; the paratypes are from level C 2 or 

 C 3, a difference of geologic level representing a very 

 long period of time, in which it is now certain that a 

 very marked progressive evolution took place in teeth, 

 skull, and skeleton. 



Our geologic leveling of the Bridger formation, 

 described in Chapter II, has therefore not only 

 afforded us the means of determining the evolutionary 

 succession of the species of titanotheres but, if the 

 localities of the types were properly recorded by the 

 authors, it has enabled us to separate many er- 

 roneously associated type specimens. The geologic 

 levels of the materials recently acquired by the 

 American Museum have been ascertained precisely; 

 on the whole, the successive species correspond very 

 closely with the successive levels — that is, in no case 

 have different species in the same line of descent been 

 found at the same level, although species in different 

 lines of descent (that is, in different genera) are found 

 in analogous stages of evolution. 



