OF THE MESOZOIC MAMMALIA. 
217 
mens of Cte^iacodon discovered by Professor Marsh, the true molars are unfortunately 
missing, although the premolar series is complete, (fig. 8, 6, c). The five teeth pres- 
ent increase from before backward. The first and second are of the Bolodon type, i. e., 
with a subcircular crown supporting three conic tubercles; the third is sub-trenchant; 
the fourth and fifth are trenchant with deeply indented borders. Behind the fifth is 
a space for two molars, (p, 333 Am. Jur. Mamm.^) 
Is there a family relationship between Bolodon and Plagiaulax ? In comparing 
this series with the corresponding teeth of Bolodon, we observe that the Bolodon pre- 
molars decrease rapidly in size from the first to the third ; secondly, that the succeed- 
ing teeth increase rapidly from the fourth to the seventh ; third, that the third tooth 
behind the diastema in the Gtenacodon maxilla is large and trenchant while, in Bolo- 
don it is tritiibercular and very small; the fourth tooth behind the diastema in Gten- 
acodon is high, trenchant and has an indented upper border, while the similar tooth 
in Bolodon is a lo w crown bearing six conical tubercles. Dr. Lemoine’s discovery 
renders it probable that the upper molars of Plagiaulax had three rows of cusps, but 
111 default of positive evidence upon this point, we must compare the Bolodon maxil- 
lary molars with those of the Plagiaulax mandible. They are very widely distin- 
guished from each other (a) by the presence in the former of a strong cingulum which 
embraces three fourths of the crown; (b), by the double row of distinct conical 
tubercles with striate sides, arranged in parallel lines, and separated by a well worn 
median groove. These cannot be mistaken for the crenate margin of the basin- 
shaped molars of the Jurassic Plagiaulax; they belong rather to the Tritylodon type 
of molar with two rows instead of three. I had previously supposed that the edges 
of lower premolars of the Plagiaulax type might fit in these grooves, but this hypo- 
thesis is disproved by the above discovery. The two anterior premolars of G. potens 
are, it is true, similar to those of Bolodon, but beyond this there is no homology in 
the maxillary dentition of these genera. The dentition of Bolodon suggests a fore- 
and-aft grinding motion ; that of Plagiaulax a vertical motion of the mandible. 
The determination of the dental formulte in these genera, at the present time, is 
largely an arbitrary matter. Judging from the natural division of the tooth structure 
the formula of Gtenacodon is i} eg pt m|, differing from the Bolodcm formula as given 
below. The wide separation indicated by these numerous diverse characters over- 
weighs the affinity suggested by the likeness of the two premolars and makes it 
necessary to place Bolodon in a distinct family. 
MENISCOESSUS, Cope, 1884. 
This genus, from the American Cretaceous, is much larger than the Mesozoic or 
Eocene Plagiaidax. It is represented by a single molar and premolar tooth probably 
belonging to the maxillary series. The molar tubercles are arranged in three rows. 
* Professor Marsh places Allodon in the Plagiaulacidse. 
