P IIOZ P U G LOD( )N AT ROX. 
255 
The ventral surface of the centruiu bears two bhuit ridges, se[)arated by a narrow 
groove. 
Comparison of these vertebne with those of Frotocetus ataims shows that a 
considerable alteration has taken place, and that there is much less similarity to 
the vertebne of a carnivore. The atlas of Frotocetus is unknown, but in the axis the 
peg-like odontoid and the relatively long centrum are characters approaching those 
found in the Carnivora ; on the other hand, in Frozeuglodon the odontoid has already 
become short and blunt, and the centrum is much shortened, as also are the transverse 
processes; at the same time, the high massive neural spine is retained. In the third 
cervical, the great shortening up of the centrum in Frozeuglodon compared with the 
same vertebra in Frotocetus is most striking ; in this case also the transverse processes 
are reduced, while the neural spine is moderately developed. Comparison with the 
corresponding vertebrae of ZeugJodon shows that the present species approaches much 
more nearly to that genus than to Frotocetus. The atlas is similar to the imperfect 
specimen figured by Miiller (pi. xiii. figs. 1, 2), except that the ventral bar is not so 
thick ; in both there is a hypapophysis. The odontoid process is more rounded than 
in Zeiiglodon, its upper surface showing very little trace of flattening, and therefore 
differing widely from that of Z. zitteli as described by Stromer ; the neural spine 
overlies the atlas, as described by Lucas. The third vertebra is shortened to about 
the same degree as in Z. osirls, and possesses a fairly developed neural spine, said to 
be absent in the corresponding vertebra in Z. cetoides. 
This genus in the structure of the skeleton, so far as known, does not throw any 
new light upon the relationships of the Zeuglodonts to the Creodonts on the one hand 
or to the Odontoceti on the other ; and its chief interest consists in its almost exactly 
intermediate position between Frotocetus and Zeuglodon., both in structure and in time. 
Prozeuglodon atrox, sp. nov. 
[Plate XXL ; text-figs. 80-83.] 
Type Specimen. — A nearly complete skull with right ramus of mandible (described 
above, and figured on PI. XXI.); Geological Museum, Cairo. 
This is the only species at present known. The length of the skull is CO cm. ; 
the length of the upper premolar series I8'2 cm. 
Form. & Loc. — Birket-el-Qurun beds (Middle Eocene): a valley about 12 kilometres 
W.S.W. of the hill called Gar-el-Gehannem (see map in Introduction). 
C. 9319. Skull with right ramus of mandible. The palatal region and base of the skull are 
imperfect. In the upper jaw the incisors and canine are represented by their alveoli 
only ; the first premolar is just appearing, wdiile the second, third, and fourth have high 
secant crowns, the two latter possessing three roots. Only one molar is in sdu., and it is 
