10 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I49 



capromys, and Hexolobodon), represented on Hispaniola only by 

 the well-differentiated Hexolobodon phenax, has its greatest diversity 

 on Cuba with two genera, weakly differentiated, and five species. 4 



Available evidence suggests that the Plagiodontia group has under- 

 gone a relatively old radiation on Hispaniola but is not known with 

 certainty to have dispersed from the island by natural means, whereas 

 the Capromys group has undergone a relatively young radiation on 

 Cuba and has dispersed widely from that island at least in part by 

 natural means. 



If Quemisia is in fact related to the capromyids, then it would 

 suggest that the group has been in the Antilles longer and has under- 

 gone more extensive evolution there than previously would have been 

 supposed. It is not impossible that Antillean caviomorphs have arisen 

 from fewer, possibly older, invasions than their current taxonomic 

 separation would imply, and that the descendants of a single invasion 

 could in some cases have diverged to the familial level after invasion. 

 On the basis of present evidence one can speculate at least that 

 Quemisia, when better known, will provide an evolutionary bridge 

 between Antillean capromyids and heptaxodontines. 5 Naturally, 

 since all known forms are of Quaternary age, they must be re- 

 garded as collateral members of an adaptive radiation, none of which 

 is ancestral to another, and all of which have evolved at differing 

 rates both in the relation of one lineage to another and of one struc- 

 ture to another in a single lineage. Regarded as divergent products 

 of a single adaptive radiation, the heptaxodontines are relatively 

 highly evolved in terms of hypsodonty and enamel configuration, but 

 conservative in retention of P 4 diphyodonty and root formation in 

 P 4 -M 3 , whereas the capromyids are more conservative in degree of 

 hypsodonty and enamel modification, but advanced in the suppression 

 of P 4 and evergrowing cheekteeth. Quemisia presents a melange of 

 highly evolved and conservative characters. If the relationships sug- 

 gested here are real, the waif which gave rise to the Antillean 

 capromyids and heptaxodontines must have dispersed from the main- 

 land prior to the suppression of P 4 . According to the interpreta- 



* Schaub (1953, pp. 396-397) distributes the genera discussed in this paragraph 

 among four families, an arrangement which I am wholly unable to accept, but 

 the analysis of which is outside the scope of this paper. 



6 Wood and Patterson (1959, pp. 325-326) have utilized the lateral process of 

 the supraoccipital in drawing the Echimyidae and Capromyidae together but have 

 pointed out with regret the strong development of the process in Elasmodontomys. 

 Is it possible that the lateral process in Elasmodontomys is of more profound 

 significance than merely another regrettable instance of rodent parallelism? 



