DEVELOPMENT OF THE SKELETON OF THE TUATABA. 35 



is considerable variation. Boulenger has called attention l to the fact that " the 

 anterior extremity of the plastron overlaps the posterior extremity of the sternum," and 

 concerning this, two specimens we have examined yield interesting results. In each of 

 them there are two free gastralia in front of the first that is attached to the ribs ; and 

 while in one example (that possessed of four pairs of sternal ribs) these are short and 

 median, in the other (that in which the last " cervical " rib is sternal) the anterior 

 gastralium is represented by a symmetrically-arranged pair of rods. 



Giinther originally described each gastralium as of three elements, and their natural 

 relations are generally those indicated in PI. II. fig. 10, no. 7. Not unfrequently union 

 in the middle line, as figured and described by him, may occur, and it would seem to 

 involve a forwardly-directed spur on the median element which is generally present 

 (of. PI. II. fig. 10, no. 11). Much interest attaches to Giinther's description of an 

 exceptional specimen, in which for the rib-connected gastralia the lateral elements were 

 attached to the central by transverse "joints," instead of being in oblique juxtaposition 

 with them as for the free gastralia and as is more generally the case throughout the 

 entire series. The fact that, in most specimens we have examined, individual gastralia 

 of both the attached and free order show both "jointing" and juxtaposition, and that 

 while in most cases one or two such "joints " may be present only on one side there 

 being no regularity recognizable, proves that no physiological significance is to be 

 attached to these variations. 



Turning to the embryo, we find that the gastralia arise at Stage S, within previously 

 differentiated fibrous tracts of the subcutaneous tissues of the body-wall (g.t., PL II. 

 fig. 9), by a process of calcareous deposition, which proves them to be wholly 

 membranous in origin. Their connection with the myocommata of the body-wall is 

 fully established later, and with Gadow 2 we have failed to detect any traces of a 

 cartilaginous origin, believed by Schneider to be the case for the Crocodilia. Figs. 8 

 and 9 represent (8) the anterior (third to fifth), 9 the posterior (16th to 18th and more 

 highly magnified) gastralia of an example at Stage S. It will be observed that the 

 median element is present only anteriorly, and that while for two of the three segments 

 figured comparison with the adult shows its lateral cornua to be partly represented by 

 a series of minute calcifications, for the anterior segment it is composed of two pieces. 

 Anteriorly the lateral elements are seen to be fully formed, but on passing to the posterior 

 series two of the three figured are observed to be represented by a linear series of 

 distinct calcifications, the median element being undeveloped. The fact that in the 

 adult the gastralia which occupy the positions of those here figured are alike 

 symmetrical and only trisegmental, proves that with advancing development a union of 

 the calcifications such as are here figured must occur, and that the median elements 



1 Boulenger, G. A.: Brit. IVIus. Cat. Chelonia, Rhynchoceph. & Emydos. 1889, p. 1. 



2 Gadow, H. : Morphol. Jahrb. Bd. vii. 1882, p. 77. 



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