210 
ON THE STEUCTURE AND CLASSIFICATION 
tlie molars, accompanied by a multiplication of the tubercles into two thickly studded 
rows, separated by a groove. In the Jurassic species, the molar crowns form a sub- 
circular basin which shows no signs of fore-and-aft wear, the marginal tubercles being 
mere crenations ; in the later forms, the basin is converted into a groove by the ap- 
proximation of the sides, the tubercles become distinct, and the groove shows fore-and- 
Figi'ke 8. a, Ivcft lower jaw of Ctenacodon saralus, Marsh, inner view, three times natural size: 
b. Right upper jaw of C. polem, inner view x4. c. The same seen from below, a. First premolar, b, fourth 
preiiiolar as interpreted by Professor Marsh. After Marsh. 
aft wear as in the Bolodon molars. These stages, already partly described in the 
valuable memoirs of Cope and Lemoine, may here be presented synoptically : 
Jlihdus, ' Cope. 
» T pm I j 
The first and second premolars 
entirely wanting; the third is 
rtidinienlary ; the fourth has 
about l.'i obli(|UC grooves. The 
first molar is narrow and elongate 
with three internal and five exter- 
nal tulicrcles. The second molar 
has two tubercles on the inner 
row and four upon the outer. 
There is a well-worn groove be- 
tween the rows of tubercles. 
Neoplagiaulax, Lemoine. 
The condyle is elevated above 
the angle and transversely ex- 
tended. The fourth premolar 
alone remains and is marked by 
fourteen deep oblique grooves. 
The molars are very elongate in 
TOj, there are six tubercles in 
the inner, and nine in the outer 
row. In m 2 there are 3 or 4 in 
the inner, and 5 in the outer row. 
Thylacoleo, Owen. 
The fourth (?) lower preraolar 
only is preserved, with smooth 
sides and well marked grooves at 
its base. (Falc. Pal. Mem. Plate 
34, fig 9). 
N. B. The position of 'Ihylacoleo 
in this series is very doubtful. 
The structure of the upper molars has an 
I j"* “f Phgiaulax and Bolodm. Dr. Lemoine'' in 
: “T"* ‘wo molars with a triple row 
01 ulmrclcs, 1 ut otlierw.se so closely resembling those belonging to the ma^ible 
that he conjectu red they belonged to the maxillary series. In the maxillary s^S 
‘American Naturalist, 1884, p, 370. 
‘ f.tude sur le Neoplagiaulax, etc., PI. VI, fig. 17. 
