Pro/. 0. C. Marsh — American Jurassic Mammals. 243 



Among the separate upper jaws found in the Jurassic of England 

 were two or three described by Owen, under the generic name 

 Bolodon, but with no suspicion that they were in any way related 

 to Plagiaulax. From American deposits, also, somewhat similar 

 jaws were obtained more recently, and as they were apparently 

 quite distinct from Bolodon, they were described by the writer as 

 representing a new genus, Allodon. The molar teeth in one 

 specimen resembled those of Plagiaulax, and the writer in his 

 description expressed the opinion that Allodon should probably be 

 placed in the PlagiaulacidcB. A natural inference was that Bolodon 

 was the upper jaw of Plagiaidax, and Allodon, of Ctenacodon. How- 

 ever this may be in regard to the European forms, the specimens 

 now known make it clear that the American genera are quite distinct. 



The molar teeth of Allodon and Ctenacodon are of the same general 

 type, and it is still difficult, if not impossible, to distinguish them 

 when detached from the jaws. The premolars, however, and 

 especially the incisors, differ in the two genera, and when well 

 preserved may often be separated with certainty. 



Allodon. 



In Allodon, the superior dentition on each side appears to be as 

 follows : — Incisors 3 ; canine ; premolars 5 ; molars 2. 



The lower dentition is uncertain, but is probably the following : — 

 Incisors 1 ; canine ; premolars 4 ; molars 2. 



The upper molar series in the type specimen of Allodon is well 

 shown in Plate VI. Figures 1 and 2. The five premolars have 

 tuberculate crowns, and all appear to be inserted by two fangs. 

 The first and second have each one external, and two internal cones. 

 The third premolar has a small additional cusp. These three pre- 

 molars diminish in size from before backward. The next premolar, 

 or fourth, is much larger, and has its crown flattened on the inner 

 side. There are three tubercles on the outer border, and four on 

 the inner margin. The fifth in the series is still larger, and has 

 a more rounded crown. There are three lobes on the outer side, 

 and the same number on the interior face. 



The two true molars have low crowns, which are divided into an 

 outer and inner half, by a deep- worn groove. Each half bears three 

 low tubercules, of nearly equal size. The last molar has its longi- 

 tudinal groove on a line with the inner margin of the other teeth. 



The superior incisors of this genus now known are represented 

 in the detached premaxillary of Allodon fortis, Plate VI. Figui'es 

 7-10. The first incisor was very small. The second was the main 

 front tooth, much larger than the third. In the type specimen of 

 Allodon, represented in Plate VI. Figures 1 and 2, no suture is 

 visible behind the first small tooth (a), hence this may possibly be 

 a weak canine instead of the third incisor. In Allodon fortis, 

 Fig. 7, and also in the type of Bolodon, Owen, the suture between 

 the premaxillary and maxillary is distinct. 



The large second incisor of Allodon is a peculiar tooth, and was 

 evidently exposed to the full wearing action of a strong lower incisor, 



