436 M. Serres on Glyptodon ornatus. 



and quite at the extremity of the lower margin of the ischium, 

 there is a small, smooth articular surface, about 1 centimetre in 

 diameter. 



The outer surface of the ischium is convex from above down- 

 wards, and slightly concave from before backwards. The inner 

 surface presents the opposite arrangement. 



At the lower posterior part, in the immediate vicinity of the 

 lozenge-shaped surface of the lower margin of the ischium, the 

 inner face also presents a large synarthrodial surface, of a some- 

 what pyriform shape, 2 centimetres broad in front, narrower 

 behind, and about 6 centimetres in length. 



The ischium, regarded as I have just described it, measures 

 25 centimetres in length from the margin of the cotyloid cavity 

 to the carapace. But it is easy to see that it is really composed 

 of two different osseous pieces. The bad condition of the parts 

 has prevented my investigating this curious point of osteogeny 

 so completely as I could have wished. It is necessary to take 

 into consideration the difficulties inherent in such observations 

 on broken and reconstructed bones. Moreover the individual 

 itself was by no means favourable for investigations of this 

 nature, age having already produced in it certain synostoses 

 which were evidently accidental. 



Nevertheless I think it very probable that we may regard the 

 posterior region of the ischium as independent of the region 

 united to the sacrum, as a sort of epiphysis of which the anchy- 

 losis is very late, if indeed it ever takes place. I may, moreover, 

 remind the reader that in the young Armadillo the ischium 

 likewise presents at its extremity a distinct osseous piece, the 

 nature of which must be the same. 



In our specimen, at about three-fourths of the length of the 

 ischium, we see very distinctly a solution of continuity parallel 

 to the base of this bone, and consequently nearly vertical, which 

 separates it into two regions. This solution of continuity is an 

 articulation. This may be very well seen towards the upper 

 margin of the ischium, where the articular surfaces of the two 

 pieces become considerably widened in consequence of the thick- 

 ness of that margin. They are there nearly 2 centimetres in 

 diameter, and present pretty nearly the aspect of the epiphysary 

 surfaces of young bones. 



This curious synarthrosis is very distinctly visible on the two 

 sides. Below, it pretty nearly cuts in half the lozenge-shaped 

 and pyriform articular surfaces of which I have already spoken as 

 occurring on the inner face and inferior margin of the ischium. 



From this arrangement it follows that, in Glyptodon ornatus, 

 the sacro-iliac region of the pelvis was not united in a perfectly 

 rigid manner to the carapace by the ischium. It is evident that 



