446 MILLER AND GIDLEY." SUPERGENERIC GROUPS OF RODENTS 



Family Petromyidae 



In general resembling the Ododontinae but crown of each cheektooth 

 margined by two elevations on the protomere, these elevations probably 

 resulting from the unusual obliquity at which the teeth appear to be set. 

 The teeth are rooted, strongly hypsodont; the enamel pattern consists 

 of two transverse lobes united by a median isthmus, the outer edges of 

 the lobes becoming joined in the upper teeth when worn. No speci- 

 mens examined.^ Recognized as a family by Tullberg, partly on whose 

 authority we continue to treat it as distinct. The characters of the teeth 

 indicate important mechanical peculiarities of the chewing apparatus. 

 The enamel pattern appears to be of a type which could be directly de- 

 rived from that present in the relatively low-crowned molars of Erethizon 

 and the Oligocene African Phiomys. 



Petromys, South Africa: Recent. 



Family Myocastoridae 



In general like the Erethizontidae but upper zygomatic root over 

 middle of toothrow, and cheekteeth with structure paralleling that pres- 

 ent in Castor; lateral process of paroccipital large, projecting freely 

 above base of greatly elongated paroccipital process; in living species 

 external form modified for aquatic life. 



Myocastor and related fossil genera; South America; Miocene to 

 Recent. 



Family Thryonomyidae 



Like the Myocastoridae but cheekteeth with structure paralleling 

 that present in some of the Echimyinae, and lateral process of paroc- 

 cipital small, closely applied to base of moderately large paroccipital 

 process; external form not modified for aquatic life. 



Thryonomys; Africa; Recent. 



Family Dinomyidae 



Like the Echimyidae but cheekteeth combining a multilaminar 

 structure with excessive hyposodontj^ (parallel: Castor aides); so far as 

 known the external form is robust, terrestrial. 



South America and the Greater Antilles; Miocene to Recent. In- 

 cludes the living Dinomys and the extinct genera Amblyrhiza, Briaromys, 

 Discolomys, Elasmodontomys, Gyriahrus, Megamys, Neoepihlema, Olen- 

 opsis, Potamarchus, Tetrastijlus. 



Family Cuniculidae 



Not essentially different from the Dinomyidae, but the jugal and part 

 of the maxillary are expanded to form a conspicuous cheekplate, the 

 surface of this becoming excessively j'ugose in adult ; cheekteeth strongly 



' Mr. Oldfield Thomas has kindly sent us photographs of a skull in the British 

 Museum (No. 4.2.3.98). 



