no REVISION OF THE PELYCOSAURIA. 



The fifteenth vertebra resembles the preceding. The inferior ridge here reaches 

 to the anterior edge of the centrum. The imperfect spine of a mid-dorsal, No. i 

 Universit)' of Chicago, is 872 mm. high. 



The sixteenth to the eighteenth vertebrce (posterior dorsals). The articular faces 

 of the centrum are more nearly circular than the preceding, the anterior face having 

 lost in large measure the wide inferior face for the intercentrum. The keel is still 

 apparent, but is not so thin, the sides of the centrum not being sharply contracted 

 immediately below the notochordal canal, but contracting gradually so that the section 

 of the vertebrje at the middle is rather a rounded wedge than a cylinder with narrow 

 ridge below (see figs. 24 and 25). On the eighteenth vertebra the inferior ridge sup- 

 porting the transverse process has extended so far forward as to join the anterior 

 articular face of the centrum. These vertebrae show a gradual elongation on the 

 bottom line. The perfect spine of a posterior dorsal of No. i is 863 mm. high. 



On the nineteenth vertebra (first lumbar) the face for the capitulum of the rib 

 suddenly appears on the edge of the centrum. The sudden removal of the head of 

 the rib from a position between the centra and articulating by cartilage with the end 

 of the intercentrum to an articular face on the anterior edge of the centrum is a ver^- 

 characteristic feature in the vertebral column. In one or two species there is a single 

 vertebra which shows an intermediate stage, but in most the change is as sudden as 

 the change of position of the capitulum in the Crocodilia. The vertebra which shows 

 this change has been rather arbitrarily assumed as the first lumbar ; there is reason to 

 believe that the first free rib was attached somewhat farther back. 



The tzventieth resembles the nineteenth except that the face on the edge of the 

 centrum is larger. After the twentieth there is a break ; the next two are free from 

 the matrix and do not connect by a perfect fit with either the anterior or the posterior 

 series, but there is little doubt that they belong in the position assigned to them. ^ 



The tiventy-first to the tzventy-seventh are reckoned as posterior lumbars. The 

 centra gradually shorten on the bottom line until the posterior ones are much shorter 

 than high. The articular faces are nearly circular. The bottom line and the sides 

 are deeply concave ; a deep depression on either side of the median line below throws 

 the bottom line into prominence as a rounded ridge. The articular faces for the 

 capitulum and the tuberculum gradually approach each other until they unite in one 

 face and become much reduced in size. In the posterior dorsals the neural spmes 

 begin to grow shorter and to curve to the rear and in the lumbars this continued until 

 on°the posterior lumbars and the sacrals the spines are not over 100 mm. in height. 

 The spines rise from much farther back in the lumbars than in the dorsals so that the 

 axis of the spine is almost over the posterior zygapophyses ; this results in or accom- 

 panies the elongation of the anterior lumbars and causes a much greater distance to 

 inter\^ene between the pre- and postzygapophyses. Plate 12, fig. 10, shows three 

 posterior lumbars of No. i University of Chicago, giving an idea of the shape of the 

 centrum and the ribs. 



The tu'cntv-eighth and tiventy-ninth are the first two sacrals and termmate the 

 series in this specimen. The sacrals are best described from the next specimen. 



No. 4040 Am. Mus. The anterior portion of the column is missing in this 

 specimen, but it is complete from the ninth presacral to the ninth caudal. The ninth 

 presacral (corresponding to the eighteenth vertebra of the preceding number) has a 

 face on the anterior edge of the centrum for the capitulum of the rib, so the lumbar 

 series begins one vertebra earlier. 



