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rate it will be an advance over a mere expression of opinion — since 

 the observations here recorded are based on actual specimens. 



No reference will be made in the present paper to the ultimate 

 origin of the ribs of vertebrates since the writer has nothing to add 

 to Baub's conclusions on the subject. The subject for discussion is 

 rather the interpretation of the sacral ribs as we find them developed 

 among vertebrates. 



The earliest and most primitive vertebrates which possess sacral 

 ribs are the Branchiosauria among which there is a single pair as is 

 abundantly proven by the studies of Credner, Fritsch, von Ammon, 

 Thevenin and the writer on these forms from the Carboniferous of 

 Saxony, France, Bohemia, Germany, and North America. Lying along 

 the entire extent of the dorsal vertebrae and extending far into the 

 tail is a series of ribs which are short and stumpy. The sacral ribs 

 in the Carboniferous forms are already somewhat enlarged. Whatever 

 be the factor which develops the sacral rib certain it is that in these 

 early branchiosaurians we have evidence of the primitive condition of 

 the vertebrate sacral rib in which one of the series is localized to 

 function as a support for the pelvic girdle. This condition is readily 

 traceable among the descendants of the Branchiosauria, the Caudata. 



This condition is found not only among the branchiosaurians but 

 holds true as well for the Microsauria. In the latter case, however, the 

 caudal and posterior dorsal ribs are not so well developed as in the 

 Branchiosauria. The sacrum is provided in the microsaurs with but 

 a single pair of ribs and it is in a more highly specialized condition 

 than in the Branchiosauria for the sacral rib has assumed a more 

 stout form and is more intimately connected with the pelvis. In both 

 branchiosaurs and microsaurs the sacral ribs lie loose in the flesh. 



Among the true reptiles the sacral ribs assume a more complicated 

 structure and are more difficult of interpretation than in the simpler 

 Branchiosauria and Microsauria. In the Isodectes punctulatus Cope, 

 the oldest known reptile, the sacral ribs are two pairs and both join the 

 pelvis. In this case there are present lumbar and caudal ribs but of 

 a different form from the sacrals which are, apparently, quite heavy. 

 In the peculiar proganosaurian, Mesosaurus brasiliensis McGregor, the 

 sacral ribs are distinct. In none of the lizards nor their allies have 

 sacral ribs been determined although Siebenrock found vestiges of the 

 sacral ribs in minute ossific elements in certain lizards. A close study 

 of many embryos of the Lacertilia, of an age after the formation of 

 the ossific centers of the skeleton, has not resulted in the discovery 



