(ZyntAJ, yiaX*.CA.,AJ<^''iU', O-M. J 7 



lo. 



1 883-] 



Heterogenetic Development iit Diaptomiis, 



381 



almost all their special characters at once, in the White river. 

 The Hystricomorpha, whose home is in South America, are un- 

 known in North America below the Loup Fork or highest Mio- 

 cene, where Leidy identified a true porcupine, Hystrix venustus. 



Many of the extinct genera stand in evident genetic connection 

 with existing forms. The Miocene Castors doubtless include the 

 ance'stor of the modern beaver. The Ischyromys is a primitive 

 type of the Sciuridae, and Gymnoptychus connects it directly 

 with the existing forms by the character of its molar teeth. 

 Eumys is the primitive form of Hesperomys, as Paciculus is of 

 Sigmodon, Entoptychus and Pleurolicus are the near ancestors 

 of the Geomyidse of the Pliocene and present periods. Palaeo- 

 lagus, Panolax and Lepus form a direct genetic line. The ancient 

 genera all differ from their modern representatives in the same 

 way ; that is, in the greater constriction of the skull just posterior 

 to the orbits and accompanying absence of postorbital processes. 

 This relation may be displayed in tabular form, as follows : 



Skull wider behind orbits. 



Skull narrower behind orbits. 



Postorbital 

 processes. 



No postorbital 

 processes. 



Postorbital 

 processes. 



No postorbital 

 processes. 





Castor fiber. 





Castor peninsulatus. 

 Ischyromys. 

 Eumys. 

 Paleolao^us. 



Sciurus. 







Hesperomys. 





Lepus. 





' 





None of the species of this fauna are of larger size than their 

 modern representatives. In the cases of the beaver, squirrels and 

 rabbits, the ancient species are the smaller.^ 



-:o:- 



HETEROGENETIC DEVELOPMENT IN DIAPTOMUS. 



BY C. L. HERRICK. 



IN a paper in the Report of the Geological and Natural History 

 Survey of Minnesota, the writer suggested that this genus is 

 unusually affected by changes in the environment, and an exam- 

 ple is given in the case of D. castor. The form called giganteus 

 was shown to be probably an enlarged variety of the above. In 

 a paper in the Naturalist this matter was expanded and an 

 attempt made to parallelize the two ibrms with the two 



^ For these conclusions see Bulletin U. S. Gcolog. Survey Terrs., vi, 1881, 362-3. 





