588 PROCEEDINGS OF THE NATIONAL MUSEUM. vuu 50. 



Lull observes: 



I am inclined to agree with Hatcher in considering Astrodon and Pleurocoelus 

 synonyms, hut not in the synonym of the species, P. nanus with Astrodon johnstoni: 

 * * * Pleurocoelus altus, on the other hand, is represented by but few hones, and 

 could readily have been the possessor of teeth like those of Astrodon johnstoni. * * * 

 It is therefore quite possible that Pleurocoelus altus should be considered as synony- 

 mous with Astrodon johnstoni, in which case the latter name would take precedence. 

 It eeems preferable, however, in view of the rarity of the remains, to let the matter 

 rest in abeyance until further proof is obtained. 



The materials clearly show the presence in the Arundel of a large 

 and small species of the Sauropodous dinosauria, and while I fully 

 concur in Lull's view of the continued use of all the named species, 

 I think it preferable to assign all to the genus Astrodon, which clearly 

 has priority. 



PRICONODON CRASSUS Marsh. 

 Plate 110, fig. 3. 



Priconodon crassus was founded by Marsh 22 on a single tooth (Cat. 

 2135, U. S. N. M.) (see pi. 110, fig. 3), his original description being 

 as follows: 



The existence of another herbivorous dinosaur in the same horizon of the Potomac 

 formation is indicated by a number of fragmentary remains, the most characteristic 

 of which is the tooth figured below. This may be regarded as the type specimen. 

 Although resembling somewhat the teeth of Diraconodon [Diracodon] from the Jurassic 

 of the West, it is quite distinct. It has the narrow neck, swollen base, and flattened 

 crown of that genus, but the serrated edges meet above at a sharp angle, instead of 

 forming a wide curve at the apex. The surface shown in fig. 7 [pi. 110, fig. 3, left] is 

 much worn by the opposing tooth. In figure 9 [pi. 110, fig. 3, right] the pit formed by 

 the succeeding tooth i3 seen near the top of the fang. 



Lull, in his study 23 of the Arundel vertebrates, consisting of the 

 type and subsequently discovered materials, recognized five other 

 teeth pertaining to Priconodon crassus, and a vertebral centrum was 

 questionably referred by him to this species. The latter I regard as 

 pertaining to the sacrum, and have tentatively assigned 24 it to Ornitlio- 

 mimus. 



Lull recognized the resemblance of these teeth to those of Paleoscin- 

 cus costatus Leidy. He says : 



This tooth [the type] resembles somewhat that of Palaeoscincus costatus Leidy, 

 from the Judith River beds, though the type of Palaeoscincus is slightly smaller than 

 that of the present species. The swelling shoulder in Priconodon is more prominent 

 and rounded than in Palaeoscinus, and in the latter the cusps are much sharper and 

 more prominent, though less numerous on one edge of the crown. The median ridge 

 of Priconodon is also lacking. 



In his concluding remarks Lull sa} 7 s: 



The tooth of Priconodon comes nearest Leidy's Palaeoscincus from the Judith River, 

 to which it could readily be ancestral, as the evolutionary tendency on the part of 

 the Orthopoda is to increase the number and decrease the size of the teeth. 



w Amer. Journ. Sci., ser. 3, 1S88, vol. 25, p. 93, figs- 7-9. 



« Report Maryland Geol. Survey, Lower Cretaceous, 1911, p. 208. 



" Bull. 110 U. S. National Museum, 1920, p. 142. 



