1883.] Fleterogenetic Development in Diaptomus. 381 
almost all their special characters at once, in the White river. 
The Hystricomorpha, whose honfe 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 
ancestor of the modern beaver. The Ischyromys is a primitive 
type of the Sciuridz, 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 Geomyidze of the Pliocene and present periods. Palæo- 
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 No postorbital Postorbital No postorbital 
processes, processes, processes. processes. 
MOONS ee Sek adc. c ‘Castor fiber, Bi (eT rah se ; BS E j [Castor peninsulatus. 
nea ie ee ES ` « soies as oe | chromis. 
AE teeees ....|Hesperomys. sven duksvse naat epee 
ge Paleolagus. 
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? 
‘a e d 
HETEROGENETIC DEVELOPMENT IN DIAPTOMUS. 
BY C. L. HERRICK. 
[2 Paper in the Report of the Geological and Natural History 
Survey of Minnesota, the writer suggested that this genus 1S 
aoe affected by changes in the environment, and an exam- 
i 'S given in the case of D. castor. The form called giganteus 
pas shown to be probably an enlarged variety of the above. In 
Paper in the Narurauisr this matter was expanded and an 
~—mpt made to parallelize the two forms with the two 
a O ' 
mie Conclusions see Bulletin U. S. Geolog. Survey Terrs., VI, 1881, 362-3. 
x 
