228 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Yol. V. 



tlie posterior wall. Lower molars (P.M. 2, M. 3) with two curved trans- 

 "c-erse crests." 



Of these characters, it may bo remarked, that those derived from the 

 molar teeth must be included in a diagnosis of this family, because they 

 belong to it, although they do not distinguish it from the tapiroid 

 groups ; the one character of the superior molars (see my diagnosis be- 

 'ow) which defines the family, being omitted by Professor Gill. Of those 

 derived from the cranium, the form of the nasal bones is specific or 

 generic, and that of the basioccipital, one of proportion only. The state- 

 ment as to the x>eriotic bones does not express the i)eculiar character of 

 the family, but describes the condition usual in the order. The union 

 of the maxillary and nasal bones is also found in the tapirs. The reduc- 

 tion of the superior incisor teeth is a significant character, and the length 

 of the neck may be so also. Under these circumstances, I will endeavor 

 to offer a definition free from the above objections in the present state 

 of our knowledge. At the same time I propose to make the genus 

 Hyracodon the type of a distinct family, as I have found its characters 

 to be much more distinct from those of the Bhinoceridce than has been 

 supposed. I introduce these families by distinguishing them in an ana- 

 lytical table from those of several other families of Perissodactyles, mostly 

 extinct. 



I. The external crescentoid crests of the superior molars subequal ; inferior molars with 



crescents. 



1. Inner part of crown of superior molars supi^orting tubercles Chalicotherudce. 



2. Inner part of crown of superior molars supiiorting cross-crests.. .Palceotheriidm. 



II. Exterior crescentoid crests of superior molars subequal; inferior molars with 



cross-crests. 



3. Superior molars and premolars alike and with cross-crests Tainridce. 



III. Anterior exterior crescent of superior molars shortened; inferior molars with 



cross-crests ; premolars different from molars. 



4. Superior molars with cross-crests ; a diastema Loplnodontidce,. 



5. Molars as above; no diastema TapinilUdw. 



IV. Anterior exterior crescent of superior molars much reduced ; inferior molars with 



cross-crests ; superior molars and premolars alike, with cross-crests. 



6. Mastoid bone forming j)art of the external wall of the skull ; no postcotyloid 



tuberosity of the mandible; neck elongate Hyracodontidw. 



7. Mastoid bone excluded from the walls of the skull by the contact of the 



occipital and squamosal; a postcotyloid tuberosity of the mandible; neck 

 short Ehinocendce. 



The genera of Chalicotlieriidw are as follows: Eocene: Limnoliyus 

 Leidy; Palceosyops Leidy; Leuroceplialus S. L. & S. Oligocene: Menodus 

 Pom. 5 Symhorodon Cope 5 Bceodon Cope. Miocene: Chalicothermm 

 Kaup; Nestoritherium Kaup. 



The genera of Palwotheriidce are, Palceotlierium Cuv., Propalceotlierium 

 Gerv., Paloplotherium Ow., and perhaps some others. 



The Tapiridce include Tapirus L., Miocene to recent; Miocene^ Lis- 

 triodon Meyer, Tapiravus Marsh, 'A»c/wsofZow Cope. 



The LopModontidcB embrace: Eocene forms, LopModon Cuv., Pacliy- 

 nolophus Pom., Hyracliyiis Leidy, Colonoceras Marsh. 



