EVOLUTIONARY SUMMARY. 187 



The skull location was on the extreme point of a divide between Hell Creek and one of its 

 tributaries, with a precipitous descent on either side, so that here again it became necessary 

 to construct a roadway along the edge of the canyon wall to the broader part of the divide above, 

 to which the wagon could be driven. The tackle and horse were used, as before, boards being 

 this time laid along the roadway, with small logs for rollers. Even with a large, wide-gage 

 wagon it was necessary to remove the wagon box, as it could not possibly contain the speci- 

 men. Three trunks of trees were laid on the fully extended wagon frame, which was backed 

 up to a convenient bank, on which the hinder end of the logs rested. It was then found that 

 the skull box was too wide to pass between the rear wheels, so they were undermined, and thus 

 lowered beneath the level of the logs. The box was then slid into position with little difficulty, 

 the hind axle was raised with a lifting jack, and earth was thrown beneath the wheels until 

 they again bore the load. The protruding tree trunks were then sawed off, the specimen box 

 was lashed fast to the wagon, and the task of loading was thus completed. The skull was 

 hauled out to Miles City, a distance of 135 miles, whence it was shipped directly to T New York. 



PROBABLE APPEARANCE, HABITS, AND ENVIRONMENT OF THE CERATOPSIA, 

 AND THE CAUSES THAT LED TO THEIR EXTINCTION. 



EVOLUTIONARY SUMMARY. 



The earliest known Ceratopsia are from the Judith River beds, and the race continues 

 upward until the close of the Cretaceous period, during which time it underwent a striking 

 evolution, largely one of size and armament. 



Of the ancestors of the Judith River forms we have no record, probably because, as Matthew 

 believes (see p. 194), they were dry-land types, notwithstanding the swamp-living habits of their 

 successors. 



The Judith River ceratopsians already exhibit the characteristic horns and frill, but the 

 relative proportions of nasal and supraorbital horn cores are the reverse of those of the Laramie 

 types. The nasal horn was the first to develop. It varied somewhat in form, being straight 

 and compressed in Monoclonius sphenocerus and curved strongly backward in M. dawsoni. In 

 the other Judith River genus, Ceratops, it seems to have curved forward instead of backward, 

 and the supraorbital horn cores, which were rudimentary in Monoclonius, are much more advanced 

 in development, though the nasal horn was in all probability still the larger. The frill in the 

 Judith River types is by no means so well developed as in the Laramie forms, and in Monoclonius 

 and Gentrosaurus consisted largely of the coalesced parietals, the squamosals taking but little 

 part in its formation, but in Ceratops, though the crest still consists mainly of the widely fenes- 

 trated parietals, the squamosals become a more prominent factor, tending toward the form 

 of the latter bone found in Torosaurus. 



There is evidence that the teeth of the Judith River forms were somewhat more primitive 

 than those of the later Ceratopsia in that the peculiar method of replacement was not fully 

 assumed in the earlier types. 



The stratigraphical range from the Judith River beds to those containing the earliest 

 Laramie type, disregarding for the moment Agathaumas, is 3,500 feet according to Hatcher's 

 estimate (p. 119). This implies a great lapse of time, during which the most striking evolutionary 

 changes occurred, so that Triceratops horridus, the earliest recorded Laramie species, exhibits 

 a marked advance in cranial features over its Judith River predecessors. Here the supraorbital 

 horn cores are vastly larger, while the nasal horn is much reduced, tending to disappear almost 

 entirely in T. obtusus or Diceratops. Correlated with this change of offensive armor is an increased 

 development of the defensive frill, which in Triceratops is no longer fenestrated, though in 

 Torosaurus it still retains large paired fontanelles in the parietal portion of the crest. 



The advance in the structure of the Laramie types seems, therefore, to lie in (1) the larger 

 size of the individuals, (2) the preponderance of the supraorbital horns over the nasal, (3) the more 

 perfectly developed parietal crest, and (4) the perfection of the type of dentition peculiar to the group. 



