62 3 Recent Literature. (January, 
Ginglymoid articulations vary also from three to none, but 
their position is always between the hindermost vertebrz. sé 
ginglymoid articulations have direct relation to the mode it 
which the neck is retracted, since they permit only movements 
of flexion and extension; thus it is easy to comprehend their 
absence among the pleuroderes, in which the retraction of the 
neck is performed by a deduction. The marine turtles have only — 
one ginglymoid articulation, and thus in this respect stand — 
between the pleuroderes and the true cryptoderes, which have 
two or three. 4 
The Trionychidæ have only one or two ginglymoid articulations, 
but their fewness is more than counter-balanced by the perfecti 
supplementary ginglymus presented by the dorso-cervical articu- : 
lation. From the possession of this peculiarity the Trionychidæ — 
constitute a type apart from other Chelonians. : 
The variations in the double-convex vertebrz in forms, evidently i 
nearly related, renders their physiological function less easy to 
determine, yet it is evident they play an important part in the re- a 
traction of the neck. The marine turtles possess but one of i 
these vertebræ, and this has but slight convexities. a 
The Chelydras, Cinosternes and Staurotypes have also but one, 
with convexities more prominent than in the marine turtles (Tha- 
lassians), and in these tortoises, especially the ftrst group, the re- 
traction of the neck is incomplete, although more perfect than in 
the marine turtles. The true cryptoderes, Testudo, Emys, Cistudo, 
Terrapene, etc., have two such vertebræ ; while the pleuroderes with 
long necks have two; those with shorter necks one. M. Vaillant 
does not venture to assign a reason for the variable position occu- 
pied by these double-convex vertebra, since it is not constant 
even in the same genus. 
M. Vaillant promises to follow up his valuable researches upon 
the hard parts by a study of the actual mode of articulation and 
of the muscles. Not only is the mode of articulation very vari- 
i 
varieties. _ : 
In most cases the atlas is distinct from the odontoid process, 
whick is united by a close amphiarthrosis to the second vertebra, 
and movements are effected by articulations which unite it to the 
three portions of the atlas, which in very old individuals is a sin- 
gle bone. In most pleuroderes the odontoid apophysis is firmly 
