232 B. NATURAL HISTORY. 
- From this point two lines of modification can be traced. The one, 
coinciding with that in which the quadrate and suspensorial bones 
are received into closer cranial articulation, is characterized by the 
wider separation of the two surfaces. The inferior becomes mar- 
ginal and sessile, remaining on the centrum; the superior rises, and 
on the dorsal region is supported on an elongate basis from the 
sides of the neural arch. Thus, in this point also, this series tends 
towards the Aves and Mammalia. The second, or special series, 
in correspondence with the liberation of the quadrate, etc., sees a 
fusion of the two articular surfaces, and their usual retention on 
the centrum. In one group (Sauropterygia) this fused basis rises 
to the top of the neural arch in the dorsal region: on the cervical 
region they are distinct. 
In the Crocodilia, the capitular articulation does not rise to meet 
the tubercular in front of the posterior dorsal region; and they are 
united and rise from the neural arch on the lumbar region. These 
two orders are otherwise allied, and form a point of connection 
between the groups defined by the characters of the rib articula- 
tions. 
In the Testudinata, the ribs are single headed as in this series, 
but the convexity is sometimes in contact with the transverse ex- 
pansion of the neural spine. There appears, however, to be no 
true articulation here, nor any diapophysis.* The space between 
the vertebral expansion and the tubercular region of the rib is 
filled by a later and distinct ossification. The capitular articular 
facets are sessile, and at the point of contact of two centra. The 
majority of this order present a special peculiarity in the expan- 
sion of the ribs into an osseous upper shield: a similar expansion 
of abdominal elements (perhaps abdominal ribs), with the claviclest 
and mesosternum (interclavicle, Parker), forms an inferior shield. 
As these characters are not developed in Sphargidide, they need 
not be necessarily regarded as ordinal. 
A similar character is to be found in the Pythonomorpha and 
Sauropterygia, whose fore limbs are specialized into swimming 
organs or paddles. Since we see this last modification of the 
truly differentiated limb to be subordinated to the characters of 
the order in the Testudinata (e. g., in Cheloniide and Sphar- 
* Ina young Testudo mauritanica the proximal extremities of the ribs are de- 
curved to their capitular articular facets, without touching the expansion of the 
neural spine (vertebral bones of carapace), and without sending tubercle or pro- 
cess to meet them. 
+ See Parker on the shoulder girdle. 
