36 PROF. G. B. HOWES AND MR. H. H. SWINNERTON 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. IT. 
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 Paleohatteria 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 Paleohatteria, 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! the number of “abdominal ribs” as regularly six for each vertebral segment 
possessed of them; and in instituting comparisons with Paleohatteria, 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 paleontology, 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) (Hatteriide, Homeosauride, Rhynchosauride, 
* Credner, H.: Zeitschr, deutsch. geolog. Gesellsch. Bd. xli. 1889, p. 330. 
