24 OSTEOLOGY OF PTERANODON. 



was short, is well supported by the material preserved in the Marsh Collection. In 

 Pteranodon sp., No. 2489, five free caudal vertebrae have been preserved, and are shown 

 from above, obliquely, in Plate XIII, figure 4. These apparently followed the three 

 primitive caudals or urosacrals forming the posterior division of the synsacrum. They 

 dwindle rapidly in size, and although badly crushed, are supposed to indicate a length 

 of tail of from 12 to 16 mm in an animal about nine-tenths the size of P. ingens, No. 1175. 

 So short a caudal member could have been of little service as a rudder, and the animal 

 must have depended largely or wholly upon other means of guiding its flight. It will 

 be remembered in this connection that other practically tailless forms of flying animals 

 have no apparent difficulty in directing their movements. Plate VI, figure 23, shows 

 the lower surface of four caudal vertebrae of another example of this genus, No. 2546, 

 about the same size as No. 2489. These vertebrae are still contained in the matrix and 

 presumably represent the middle third of the caudal series. 



The caudal vertebrae have been described by Professor Williston as amphiplatyan 

 {Kansas Univ. Quart., vol. vi, No. 1, Jan., 1897), while Professor Gadow (Amphibia and 

 Reptiles) regards the caudal vertebrae of all Pterosauria as amphicoelous. Some of the 

 larger caudals of Pteranodon in the Marsh Collection appear slightly concave at both 

 ends, but their original condition is uncertain, their form having been greatly altered 

 by pressure. It is to be expected that the character of the central articulations would 

 be somewhat obscure in the degenerate tail ascribed to this genus. 



SHOULDER GIRDLE. 



The shoulder girdle, consisting of the scapula, coracoid, and the sternum, has been 

 so fully described by Professor Williston that little further remains to be recorded about 

 the osteology of these parts. Characterizing the sternum of one of the "smaller species" 

 of Pteranodon {Restoration of Ornithostoma, Kansas Univ. Quart., vol. vi, No. 1, Jan., 1897), 

 Professor Williston writes : "It is extremely thin and [is] pentagonal in outline. Pro- 

 jecting in front of the articulations for the coracoids is a stout process, obtusely pointed, 

 and evidently directed somewhat ventrally in life. The articular facets [for the 

 coracoids] look dorsad and laterad and are gently convex from side to side and concave 

 antero-posteriorly. Just back of the articulations the moderately thickened borders slope 

 obliquely backward to the full width of the bone. The first articular facet for the ribs 

 begins at the angles and runs backward a half inch. It is of considerable thickness, 

 and may be for the attachment of the stout co-ossified rib attached to the first of the 

 consolidated dorsal vertebrae. The lateral margins of the sternum, back of the angle, 

 are thin, and have three emarginations, separating four articular projections. The three 

 posterior ones are small and pointed, and could have given attachment to only slender 

 ribs. The lateral borders are parallel with the longitudinal axis of the bone. The 

 posterior border is not preserved, but from the general resemblance to the bone in 

 Nyctodactylus, I believe that it is nearly straight, although it may have been gently 

 concav.e or convex. The bone was in all probability concave above in life." 



No perfect sternum is known to the writer, and indeed such is the tenuity of the 

 posterior border of the bone that none is likely to be found. Three more or less com- 

 plete examples have been selected from which a fairly accurate idea of the entire bone 

 may be obtained. Plate XIV, figure 1, shows the lower surface of the vertically crushed 



