924 The American Naturalist. 3 [October, 
(2). The genus is probably to be regarded as the ancestral type of 
the Dipodids and indicates an American origin for this family, being 
much more ancient than any known representative of the group in the 
Old World, which it appears to have reached by a comparatively late 
migration. Paciculus of the John Day ‘beds is a somewhat aberrant 
number of the same line. 
(3). It is not improbable that the Heteromyide were derived from 
some form related to Protoptychus, though not from that genus itself. 
(4). The Geomyidz are descended from early forms which may best 
be referred to the Heteromyide and in which the tympanics and the 
mastoids were already greatly inflated. The assumption of subterra- 
nean habits of life brought about a reduction in this region of the skull 
and led to the acquisition of the many peculiarities which characterize 
the recent pocket-gophers. Pleurolicus and Entoptychus represent 
stages in this change and are more or less directly ancestral to the 
modern Geomyide. (Proceeds. Phila. Acad., 1895.) 
Cenozoic History of the Baltic Sea.—lIn a preliminary report 
on the Physical Geography of the Litorina Sea’ Mr. H. Munthe gives 
a summary of the present saltness of the Baltic and a report of the 
present distribution of the Mollusca that concern the Litorina-sea espe- 
cially ; he then discusses the question of the distribution of the Mollusca 
during the saltest part of the Litorina-time. The report includes also 
the author’s investigations of the diatomaceous flora of the Litorina- 
sea and its rhizopod- and ostracod-faunas (on which subject but little 
has been hitherto published) and in this connection he gives briefly the 
testimony of diatoms in the hydrography of the Litorina-sea. 
From the facts presented in the communication the late Cenozoic 
history of the Baltic can be summed up in the following manner: 
A. YOUNGER GLACIAL Epocna. 
(1). Time of the younger Baltie glacier. : 
(2). Late Glacial time. The land-subsidence in Scandinavia now 
reaches its maximum during the Cenozoic period. The Baltic has the 
character of an ice-sea with Yoldia arctica Gray, etc., and is in open 
connection with the Cattegat across the northern part of South Sweden 
(Lakes Wettern, Wenern, etc.) and possibly also with the White sea 
across the Ladoga, etc. 
1 The author defines Litorina-time as that relatively salt phase of the Baltic 
Sea’s postglacial history, which was subsequent to the Ancylus time during which 
the Baltic was shut off from the ocean and had the character of a fresh-water in- 
land lake. 
