198 C. W. Gilmore — New Species of Baptanodon. 



The following- comparative measurements are interesting as 

 showing the similarity in proportions between the humeri of 

 the American and English members of the Baptanodontidae. 



Greatest Greatest Greatest 

 length width width 

 of of distal 



humerus, proximal end. 

 end. 



mm 



Type of B. reedi, Univ. of Wyoming coll. 210 mm ]40 mm 165 

 Type of B. marshi, " " " ' " 190 " 127 " 130 " 



Paratype of 0. icenicus, Leed's collection 164 " 115 " 152 " 



Judged from the limb structure alone, the pectoral extremity 

 of B. •marshi presents the most generalized features of this 

 genus, while, on the other hand, the anterior paddle of B. reedi 

 exhibits the most specialized characters. This deduction is 

 based upon the comparative breadth of the distal ends of the 

 humeri and the manner by which they articulate with the 

 epipodal elements. In the former species, as Knight has 

 pointed out, the facet for the pisiform is rudimentary, but in 

 B. reedi the humerus has widened sufficiently to admit of the 

 complete articulation of this bone. In the manner of the 

 articulation of these elements the extremity* of B. discus 

 figured by Marsh appears to be intermediate in form. The 

 limbs, at least, suggest a direct line of development, and if the 

 geological position of B. ?*eedi is correctly determined as the 

 uppermost part of the marine Jurassic, it would be quite in 

 accord with the structural changes observed in these specimens, 

 as the other species are known to have come from lower layers 

 of these marine strata. 



In Baptanodon, as in other Ichthyosaurians, the evidence 

 furnished b_y limb structure is most valuable for working out 

 their relationships, although, as Dr. Merriam has suggested in 

 discussing the limbs of the Triassic Ichthyosaurs, " they hardly 

 furnish the whole foundation for a definite classification." 



A study of the coalesced atlas and axis of B. natans, B. 

 discus, B. marshi, and B. robustus show differences of struc- 

 ture which would indicate that other characters that will serve 

 for specific separation will probably come to light when the 

 detailed structure of these several species is better known. 

 Unfortunately the missing centra of the anterior cervicals of 

 this specimen renders it impossible to compare these vertebrae 

 with the homologous parts of the other species. 



U. S. National Museum, Dec. 17, 1906. 



*As suggested in a previous paper, the paddle identified by Professor 

 Marsh as a pelvic limb is undoubtedly a pectoral extremity, and it is so con- 

 sidered here. 



