363 



of any element which might be interpreted as a sacral rib. In two 

 specimens, which happened to be the right age, the ossific growth is 

 seen to be taking place in the transverse process outward from the 

 vertebra. Chondrification stages might show evidences of the sacral 

 ribs in lizards but my researches have not been perfected along this 

 line so there is nothing to report. In the turtles there is early in the 

 cartilage no indication of other than a simple element for the sacral 

 rib and later stages cleared to show the skeleton have revealed nothing 

 but the simple condition of the rib. Young animals show the sacral 

 rib to perfection as well as small caudal ribs. 



The alligator, on which F. von Huene has recently laid so much 

 stress in his discussion of the sacral ribs ^), shows the same simple 

 condition. Stages from an inch and a half in length, when the first 

 stages of ossification set in, up. to young animals one year old, have 

 been cleared in the Schultze solution and there is not a single 

 departure from the simple condition of the sacral rib, thus giving 

 evidence that the sacral rib is none other than a modified rib localized 

 to support the pelvis. There is no indication of a diapophysis taking 

 any part in the formation of the rib as suggested by von Huene. 



Among the plesiosaurs Williston has recently described, for the 

 first time, sacral ribs which indicate simply the ordinary relation. 

 There is the usual sutural union with the transverse process of the 

 centra. In the crocodiles this process is more aborted than it is in 

 the plesiosaurs. 



It is among the Dinosauria where a peculiar condition of the sacral 

 ribs is found. In the Sauropoda and Theropoda the diapophysis has 

 grown out to function as a sort of secondary sacral rib but it has 

 nothing to do with the morphology of sacral ribs since the condition 

 is peculiar to the dinosaurs and is due to their habit of life. The 

 fact that there is a fusion between the upper part of the sacral rib 

 and the centra as observed by von Huene in the dinosaurs and croco- 

 diles does not prove that this means any other than a mechanical 

 cause and its effect. The condition is one of accident almost and is 

 to be regarded as among the class of other accidental resemblances 

 which Dean has well shown to have been too highly valued. We must 

 differentiate between parallel or accidental and congenital characters. 



Among the Phytosauria and their allies, the Aetosauria, which are 

 sometimes classed as a suborder of the Phytosauria, the sacral ribs 



1) Anat. Anz., Bd. 33, p. 378. 



