36 PEOF. G. B. HOWES AND ME. H. H. SWINNEETON ON THE 



arise antero-posteriorly. It further indicates a compound origin for all the parts, median 

 and paired alike ; and concerning the anterior paired segments, which in the individual 

 figured were single, we find that in younger individuals at the same stage they may be 

 represented by a series of elements. The details point to the conclusion that union is 

 rapid and irregular. 



A striking characteristic is this irregularity of calcification ! It leads to irregularity 

 of union ; and it is fair to assume that in this lies the explanation of the asymmetry 

 which the parts of the adult "plastron" are apt to assume. It explains the not 

 infrequent presence of but a single asymmetric element, and of so extraordinary 

 a condition as that of the triradiate or quadriradiate union delineated on PI. II. 

 fig. 10 (Stage T) the most erratic we have observed. It also renders clear the sub- 

 stitution of transverse "joints" between, for oblique juxtaposition of, the median and 

 lateral elements described by Giinther, and the co-existence of these herein recorded, 

 while it presupposes other combinations yet to be discovered. 



Interesting and unexpected as are these details ontogenetically, we have come to 

 attach a still greater importance to their probable phylogenetic significance. 



One of the most distinctive characters of Credner's Palaeohatteria of the Permian is 

 the multisegmented condition of its " abdominal ribs," which, on careful examination of 

 the originals, he has shown to be also the condition in Von Meyer's famous Protero- 

 saurus and other genera to which he refers (op. cit. p. 538). In view of the obvious 

 similarity to the embryo Sphenodon suggested, it becomes the more interesting to find 

 that whereas in Palaeohatteria, according to Credner's original description, there appear 

 to have been three rows of calcifications present for each rib, in Proterosaurus there 

 were two or three. In a later paper, however, on Kadaliosaurus, an allied genus, he 

 gives 1 the number of " abdominal ribs " as regularly six for each vertebral segment 

 possessed of them ; and in instituting comparisons with Palaeohatteria, Proterosaurus, 

 Stereosternum, and Hyperodapedon, he gives the number present in them collectively 

 as from three to six. The calcifications in all these forms are regular and structurally 

 constant, whereas in Sphenodon they are developmentally irregular and inconstant. 



Kadaliosaurus is further interesting in the light of the foregoing observations upon 

 the substernal extension of the " gastralia " in Sphenodon, as in it they are still more 

 nearly pectoral in extent and Stegocephalian. The facts suggest that the " plastron " 

 may be undergoing reduction in the living genus, a consideration which may perhaps 

 explain the irregular mode of calcification by which it is formed. 



Boulenger, availing himself of these facts of palaeontology, has made them a basis 

 of classification of the Rhynchocephalian Order to which we alluded at the outset 

 (antea, p. 3), and he has drawn a sharp distinction between the higher suborder 

 of the Rhynchocephalia vera (91. p. 171) (Hatteriidae, Homeosauridae, Rhynchosauridee, 



1 Credner, H. : Zeitschr. deutsch. geolog. Gesellsch. Bd. xli. 1889, p. 330. 



