944 THE AMERICAN NATURALIST. (VOL; XXXII. 
An inspection of the plastron of the alligator snapper 
(Macroclemys), or of the figures on page 26 of Boulenger’s 
Catalogue of Chelonians, shows that in this genus there is some- 
times present an intergular shield. This remarkable turtle, 
then, has vertebral, costal, supramarginal, marginal, and infra- 
marginal shields, a row consisting of the usual plastral shields, 
and occasionally an intergular, that is, rows of epidermal shields 
representing all twelve of the longitudinal keels of Dermochelys 
and, as I believe, of the ancestors of all the groups of turtles. 
So far as I know, there is no other turtle which shows all these. 
The marine turtles are not far behind, since they present traces 
of all the keels, except the supramarginals. 
The number of epidermal shields and of the hypothetical osteo- 
dermal plates belonging to each keel of the thecophorous turtles 
is, of course, much smaller than in Dermochelys, about one in 
each keel of the carapace for two vertebrz and pairs of ribs. 
An examination of the bony plates which form the armor of the 
sturgeon reveals some characters in common with those which 
we suppose once belonged to the turtles. They are broad- 
based, rise into a backwardly directed spine, and in number are 
about one-half as many as the ribs and vertebrze which underlie 
them. On the tail of Chelydra, a cousin of Toxochelys, the 
dermal bones which produce the serrations of that tail fall 
in number considerably below the vertebrze on which they 
rest. 
Reflection on the early state of the Chelonian armor has 
led me to study the condition of corresponding structures in 
Sphenodon, that reptile whose position lies so close to the base 
of the reptilian stem. 
Many reptiles, as is well known, possess longitudinal rows of 
enlarged scales, especially one which forms a crest along the 
dorsal mid-line ; and it occurred to me that possibly Sphenodon 
would show not only this but traces of other keels. What I 
find is as follows: On the dorsum of the tail there is a row of 
quite large horny tubercles, which resemble quite closely those 
seen on the tail of Chelydra. In none of them, however, do I 
find ossifications.1_ If any such have ever been present, they 
1 Günther, A. Philos. Trans. Roy. Soc., vol. clvii (1867). 
