346 Mr. O. Thomas on some 



kaolin crystals &c., while Huxley definitely called these 

 bodies, and these only, coccoliths. Since the above description 

 of their reproduction settles them finally to be self-contained 

 organisms, there arises tlie necessity for a true generic and 

 specific name. Here, however, the differentiation of species, 

 if they exist, is not possible, since they are so minute that 

 different illuminations make them appear entirely different 

 in contonr and markings. Hence I propose for all the forms 

 hitherto described, recent and fossil, the one name CoccoUthus 

 oceanicus, mihi. 



P.S. — By a strange oversight I missed Carter's paper above 

 referred to till my paper was printed. He considers cocco- 

 liths to be the cells of Mehbesia living separately, and calls 

 the oval and round cyatholiths respectively M. unicellularis 

 and M. discus] but as the discolith is undoubtedly the adult 

 form, it does not seem advisable to apply either of these 

 specific names, clearly defined for a definite thing in one stage, 

 to the same thing in another stage. 



XLIII. — Descriptions of some new Neotropical Muridse. 

 By Oldfield Thomas. 



The British Museum has recently received two small collec- 

 tions of rodents from Peru and Venezuela, among which are 

 several new species ; and in working these out the oppor- 

 tunity has been taken of re-examining and, where necessary, 

 describing the species contained in the general collection of 

 South- American Muridae. Among others, the two series from 

 Peru obtained by Messrs. Stolzraann and Jelski, and worked 

 out by me in 1882 and 1884*, have proved to need a con- 

 siderable amount of revision in their determinations. 



The first to be described represents a new genus, and is 

 again the discovery of Mr. J. Kalinowski, the finder of the 

 interesting fish-eating rat described last year "j". 



Neotomys, gen. no v. 



General form much as in Sigmodon ; ears broad and 

 rounded ; fur very thick ; tail short ; thumb with a nail. 

 Skull with the nasals much expanded anteriorly ; inter- 



* P. Z. S. 1882, p. 98, 1884, p. 447. 

 t P. Z. S. 1893, p. 337. 



