260 Bulletin American Museum of Natural History, [^'ol. XXVII, 



creases from the front backward and converges sliffhtlv toward the mesial 

 line, features which are emphasized in Xenotherium. 



The Eeinaceoidea. 



THE LEPTICTIDM, ERINACEID.E, DIMYLIDM. 



The most primitive representatives of this group are the Eocene and 

 Oligocene Leptictidte. They have already lost one upper and one lower 

 incisor, except Idops acntidens, which retains three small lower incisors 

 (Matthew, 1903, p. 207, fig. 5); but in the form of the cheek teeth they are 

 much more primitive than the Erinaceidae and some of them retain the minute 

 pml, which are lost in most Insectivores. The molars in PaJoeictops and 

 Idops suggest those of the Jurassic Drijolestes in their antero-posterior 

 narrowness, and sometimes (e. g., in the very small Ictops thomsoni Matthew, 

 1903.1) also in the large size and centro-external position of the paracone. 

 But in the larger species Ictops major (Douglass, 190G, pi. xxii) the molars 

 are much broader antero-posteriorly and the para- and metacones are sub- 

 equal. In all Leptictids p| are molariform. The lower molars are tuberculo- 

 sectorial with reduced paraconids and large hypoconids. In crown view the 

 inner and outer cusps are rather widely separated. The replacement of 

 the teeth takes place only after the animal has attained full adidt dimensions 

 (Matthew, 1909, p. 534). This delayed replacement is also characteristic, 

 of certain Zalambdodonts and may be a primitive Insectivore character. 



In Proterix INIatthew (1903.2), a primitive member of the Erinaceidse 

 which shows clear evidence of derivation from the Leptictida?, the molars 

 are broader antero-posteriorly and the hypocone is well developed. The 

 molars are in fact described as definitely Erinaceid, only the last molar re- 

 maining tritubercular, as it does in Ilylomys, Ncurogymnurus, and Galerix. 



In the living Erinaceidaj the antero-posterior broadening of the molars 

 is still more emphasized and they are more bimodont and omnivorous in 

 type than in the earlier forms. At the same time the fourth upper premolar 

 has acquired a sectorial postero-internal blade, paralleling that in the modern 

 Carnivora. 



The skull of Ictops (figured by Douglass, 1906, pi. xxii) offers some inter- 

 esting points both of resemblance and contrast to that of its modern relatives 

 Gymnura and Erinaceus and shows how much change may have taken place 

 since the Oligocene, even in very primitive forms. In profile the skull 

 slopes anteriorly into a long slender muzzle, much as in the primitive Cente- 

 toid Microcjale, and contrasts with the relatively straight profile and heavy 

 muzzle of Gymnura and Erinaceus. The mid-cranial region is not elongate 



