34 



THE AMERICAX X.m'A'.l /./sr SVin . WW I II. 



a small, slight process, which curves directly <.ut\var(l> ami tor- 

 wards, lying parallel with the ]X)steri()r l^ordcr of the rib, which 

 it joins again about an inch from its origin. — leaxing a space 

 filled up only by membrane. This may very possii)ly be 

 regarded as the rudiment of an eleventh rib, of which the 

 vertebral portion is wholly wanting. It is sometimes entirely 

 obsolete." 



"The latter ribs project so far backwards, that the thoracic 

 parietes are prolonged some distance behind the acetabula. and 

 consequently the femur in its normal position lies directly over 

 the last three or four ribs, and moves backwards and forwards 

 upon them. The angle of the last rib reaches within less than 

 two inches of the posterior extremity of the elongated obturator 

 foramen " {/oc. cit. pp. 145, 146). 



Marsh says that "the ribs of Hcspcroniis present no marketl 

 features to distinguish them from those of modern birds. The) 

 are composed of dense bone, but some of them contain irregular 

 cavities. The articulated vertebral ribs of Hcspcroniis rco-alis 

 are nine in number, on each side. The first three of tiiese 

 were attached to the last three cervical vertebra-, and had their 

 distal ends free. The remaining six are all well develoi)ed ribs, 

 which were connected by means of the sternal ribs with the 

 sternum " {loc. cit. p. 63). 



In comparing Marsh's figures of the sterna of Hcspcroniis 

 regalis ^\\(^ Hcspcroniis crassipcs, I find upon either costal bor- 

 der of the sternum of the first-named species but four facets for 

 articulation with the costal ribs, whereas in the latter form there 

 are Jive represented ; and from this I am led to believe that 

 there was quite as much, if not more, variation in this matter of 

 ribs among those now long extinct types as there is among their 

 existing affines. (Compare Marsh's Plates vi and vii. Figs, i, 2, 

 and I, 2 respectively.) 



Grebes, loons, and the great toothed divers of the Cretaceous 

 period all vary in this i)articular. And sometimes, too, as we 

 know, they are apt not to agree even in the number of facettes 

 on the costal border of the same sternum. Frequently the 

 number varies for the genus Urinator now under consideration, 

 of the family Urinatoridne, — sometimes in the same species, 

 but more often among different species. 



