612 The American Naturalıst. [July, 
aa, Numerous gular folds; vertebral canal enclosed by 
diapophyses and parapophyses ; 
Both coracoid and acromion; manus short; a 
coronoid process ; a dorsal fin ; Balenoptera. 
aaa, Only two gular folds ; 
No dorsal fin; an acromion ; Rhachianectes Cope. 
agaa, External ch@acters unknown; maxillary bones 
very narrow. 
Manus short; Mesoteras Cope. 
AA. Cervical vertebrze more or less coossified. 
Anterior three cervicals only united ; Paleocetus, Seeley.” 
All cervicals coössified; fingers five; no 
gular plicae; no coronoid process; Balena, Linn.” 
The genus Plesiocetus is intermediate in its characters, and 
as it is generalized in structure, it is probably the ancestral type 
from which modern Balenida have been, by’ a process of differ- 
entiation, derived. Four species have been described from 
Belgium. The largest of these, P. drialmontii Van Ben., was 
some sixty feet in length; while the P. brevifrons Van B. and 
P. affine Van B. were twenty feet and less in length. Cetotherium 
is more nearly allied to Balenoptera (the finners). The number 
of species appears to have been considerable, several having been 
described from Southeastern Europe, one from Italy (C. capellinit), 
and others from Belgium and England. Corresponding species 
have been found in the Miocene beds of the Eastern States of 
North America. The C. cephalus Cope is about thirty feet in 
in length, the head being nine feet; and its flippers short. The 
ear bulla is noticeably compressed, somewhat incurved, and with 
a nearly parallelogrammic outline from the side; (Fig. 7). The 
skeleton was found in Charles Co., Maryland. (Plate XXII) 
There have been described several species, probably of this 
genus, from the same region and horizon, of smaller size, the 
least, C. pusillum Cope, having been about fifteen feet in length. 
14 Physalus Gray. 
15 Eubalsena, Macleayius, and Halibalena Gray; Balzenula and Balzenotus Van 
16 The difference between Neobalzena Gray and this genus is not yet known. 
