MARSUPIALIA. 793 



In criticism of the above table, it may be added that the external cusps 

 stand within the border in Stypoloplms and DidelpJiodus, and are nearly con- 

 fluent in the false sectorials of Oxycena and Pterodon. One is smaller than 

 the other in Mesonyx. In Solenodon they are rudimental. 



From the above considerations it appears that the external, often minute, 

 cusps of the teeth of Insectivora are the homologues of those of true exter- 

 nal series, and do not represent an additional cingulum. Comparisons of 

 the molars of the extinct and recent forms are thus facilitated. 



Peratherium is nearly allied to Didelphys, but has not the inflected angle 

 of the mandible of that genus. The difference in dentition as above pointed 

 out consists in the elevation of the median cusps of the superior true molars 

 into Vs, and the obsolescence of the tubercles of the external series. It is 

 quite possible that these characters may not be sustained in a comparison 

 of the numerous extinct and recent species. The name Peratherium was 

 proposed by Aymard for some similar species from France, but his charac- 

 ters were derived from the inferior dentition, and were of no great impor- 

 tance, as remarked by Filhol. Thus he states that the third inferior premolar 

 is larger in the extinct than in the recent species, a character not generic, 

 and which, as Filhol shows, is not common to all the French species, and 

 -which is not found in those of America. 



Animals of this genus were abundant in North America during the 

 White River epoch of the Miocene, but I have not seen any of their bones 

 from later deposits. Those obtained by me in Colorado represent six spe- 

 cies, some of which,J have on former occasions referred to another genus 

 (Embassis). Future discovery may justify this course, but at present I 

 suppress the name.^ 



'In my account of the genus Herpetotherium, published in the Annual Eeport of the Hayden Sur- 

 vey for 1873 (p. 465), it is stated that this genus has a greater number of molar teeth than in Talpa, 

 "thus — in the extinct to ~ in the recent genus." As these figures are absurdly erroneous, it becomes 

 necessary to explain that the numbers originally given were ^-^ and ?^- The paper was printed 

 during my absence from the East, and the editor, not understanding my meaning, allowed them to go 

 to press in the erroneous form in which they stand. 



