108 BULLETIN 16 9, UNITED STATES NATIONAL MUSEUM 



of Ml large relative to talonid and those of M2.3 compressed antero- 

 posteriorly. M2 and particularly M3 reduced relative to Mi, heel of 

 M3 much reduced but with projecting hypoconulid. 



Discussion. — By an unfortunate chance the National Museum 

 material of this unusual genus does not include associated P4 and Mi. 

 Since P4 seems clearly related to Acmeodon, while Mi seems just as 

 clearly related to Didelphodus, and since the possibility of relationship 

 between Acmeodon and Didelphodus had never been suggested, I was 

 led to believe that two different animals were represented, one related 

 to each of these genera. Specimens collected since this manuscript 

 was first completed show that the P4 supposedly characteristic of 

 Emperodon and the Mi supposedly characteristic of Gelastops really 

 belonged to the same animal and this has made possible a last-minut« 

 correction in the present work. 



There is little doubt that Gelastops is related to Acmeodon, although 

 it is, on the whole, more primitive in structure. It had the anterior 

 premolars less reduced than in the latter. P4 is superficially quite 

 different in the two genera, but the differences appear to be modifica- 

 tions of the same fundamental structure, wliich is unlike that of any 

 other genera known to me. In Gelastops the paraconid is larger and 

 more internal and the metaconid is distinct, but the latter is prob- 

 ably represented in Acmeodon by the cuspule on the posterointernal 

 crest descending from the protoconid, in which case the distinction is 

 the relatively minor one that in Gelastops this cuspule is merely more 

 emphasized and shifted slightly anteriorly. The peculiar protostylid 

 is larger in Acmeodon but is also present in Gelastops. In Acmeodon 

 the two crests run into the talonid rim and the valley between them 

 into the talonid basin in such a way that the talonid is poorly differen- 

 tiated, while in Gelastops the talonid is well set off by notches, but 

 the parts seem to be entirely homologous in the two cases. The 

 large and internal paraconid and talonid markedly narrower than 

 trigonid, which so strikingly separate Acmeodon from the Leptictidae, 

 are developed in almost exactly the same way in Gelastops. 



On the other hand, there is also good evidence for the relationship 

 of Gelastops to Didelphodus. The general aspect of the jaw, canine, 

 and cheek dentition is much the same in the two genera. In Gelas- 

 tops Pi is probably more reduced and P2-4 more elevated. P4 is 

 definitely more specialized m Gelastops than in Didelphodus, having a 

 higher crown, more elevated (but little larger) paraconid, larger and 

 much more elevated metaconid, and more prominent posterior crest 

 on the metaconid. This crest is, however, distinctly present in 

 Didelphodus and in general all the structural features seem to corre- 

 spond in the two genera, the differences involving only relative 

 proportions and prominence of the various parts. 



