INTRODUCTION. 



INTRODUCTION. 



The permanent data of systematic palaeontology are the type specimen, determinate or indeterminate, the 

 type locality, the type geologic level. Descriptions, figures, opinions, inferences, phylogenetic and other speculations are 

 subject always to the fallibility of human observation and interpretation. 



Humanum est errare is a principle nowhere more repeatedly illustrated than in the literature on the fossil horses. 

 Much excuse for human error is to be found, first, in the fragmentary character of many of the types, second, in the 

 highly complex structure of the equine molar tooth, which is one of the most difficult objects to define and describe in 

 the whole field of vertebrate palaeontology because its specific characters and proportions differ at every age and plane, from 

 the summit of the unworn crown to the region of the fangs at the base of the tooth. This fact, first clearly set forth by 

 Gidley (1901, pp. 94-101), was little appreciated by Leidy, Cope, and Marsh, who together described sixty of the one 

 hundred and forty-six recorded species of Oligocene-Pliocene Equidae. The observations of Leidy especially were ex- 

 tremely accurate as well as philosophic; Cope's work while the most voluminous was the most confusing in respect to 

 specific and generic determination and terminology; Marsh's descriptive work was accurate but had little regard for 

 Leidy 's priority in many instances. 



The present review of the equine types is by no means final for the final revision and synonymy will depend upon 

 monographic research of a nature which has not yet been attempted on this group. Nor has there been time in the present 

 memoir to apply all the newer criteria of either single character genesis and evolution or proportional character evolution 

 which have gradually become apparent through the observations of Osbont. 



For historic reference, in the present revision the original author's description of each species is quoted direct, or in 

 abstract with interpolation [ ] of modern odontological terms by Osborn. Thus "(Leidy, 1SG9)" signifies that 



Leidy 's original language, definition or observations are quoted without comment, " (Leidy, 1S69, Osborn, 1918)" signi- 

 fies that Osborn interpolates [modern terms] in Leidy's description, "(Matthew, 1913, Osborn, 1918)" signifies that 

 Osborn endorses or expands Matthew's definition. 



I. Proportional Character Evolution. 



1. Head Ratios and Indices. 



1. Facio-cranial index = preorbital length X 100 4- basilar length. 



2. Dental ratio = p 2 -m 3 length X 100 -r- basilar length. 



3. Premolar-molar ratio = length p 2-4 X 100 -£■ length m 1-3 . 



4. Molar hypsodont index = transverse breadth of crown near summit X 100 -J- length of crown. 

 The above indices and ratios are among the most important. 



1. In MesoMppus the cranium is typically longer than the face. In Miohippus and all higher stages the face becomes increasingly 

 longer than the cranium. 



2. The dental ratio gives the proportion between the entire length of the grinding series and the basilar length of the skull. 



3. Premolar-molar ratio: the premolar series is shorter and the premolars are smaller in Eocene and early Oligocene horses; in 

 late Oligocene horses the premolars progressively increase in size so that they are larger and occupy more linear space than the molars. 



i. Molar hypsodont index: the length of the molar crown expressed in the molar hypsodont index is extremely important but often 

 difficult to obtain. In Eocene and Oligocene horses the width of the crown exceeds the height. In all higher stages the height progres- 

 sively exceeds the width. 



2. Limb Ratios and Indices. 



1. Metaearpo-brachial ratio = length of Mtc. Ill X 100 -r length of entire brachium (humerus, ulna-radius, manus). 



2. Metatarso-crural ratio = Mts. Ill X 100 -£- length of crus (femur, tibia-fibula, pes). 



3. Radio-humeral ratio = length of radius X 100 -5- length of humerus. 



4. •Tibio-femoral ratio = length of tibia X 100 -r- length of femur. 



5. Mtc. Ill index = least transverse diameter of shaft X 100 -5- greatest length of shaft. 



