246 On the Evolution of the Vertebrata, etc. [ March, 
II. Os quadratum NAS articulated ; capitular and tubercular rib-articula- 
tions distinct. Archosa 
Pubis and ischium united, and with little or no obturator foramen; one posterio: 
cranial arch; limbs ambulatory; a procoracoid............. 2. Theromorpha 
Ischium and pubis distinct, the latter directed abort: backwards or downwards; 
two posterior cranial arches; limbs ambulatory; no procoracoid , 3. Dinosauria) 
Ischium and pubis united; two postcranial arches; anterior sb volant 
4. Ornithosauria, 
III. Os quadratum closely united to cranial arches; but one rib-articulation. 
Synaptosauria. 
Distinct hyposternal and postabdominal bones; ribs joining each two vertebree, and 
enerally forming a carapace; one posterior cranial arch...... 5. Testudinata. 
Hyposternal and postabdominal bones not distinct; two posterior cranial arches; 
ribs attached to one vertebra; a sternum; ? no procoracoi 
6. Rhynchocephalia. 
Hyposternal and postabdominal bones not distinct ; two posterior cranial arches; 
ribs attached to one centrum; no sternum?; a procoracoid...7. Sauropterygia. 
IV. Os quadratum attached only at the proximal extremity, and more or less 
movable; ribs with one head. Streptostylica.? 
Brain case membranous in front of prodtic bone; trabecula not persistent 
Lacertilia. 
Brain case with osseous walls anterior to proötic; a scapular arch and sternum 
9. Pythonomorphas 
Brain case with osseous walls anterior to prodtic; no scapular arch nor sternum; 
trabecular. grooves of sphenoid and presphenoid bones 10. Ophidia. 
An inspection ofthe characters of these ten orders, and their con- 
sideration in connection with their geological history will givea defi- 
nite idea as to the character of their evolution. The history of the 
class, and theréfore the discussion of the question, is limited in time 
to the period which has elapsed since the Permian epoch inclusive, 
for it is then that the Reptilia enter the field of our knowledge. 
During this period but one order of reptiles inhabited the earth, 
so far as now known, that of the Theromorpha. The important 
character and role of this type may be inferred from the fact that 
they are structurally nearer to both the Batrachia and the Mam- 
malia than any other, but present characters which render it prob- 
: res that all the other reptiles, with possibly the exception of the 
1 This definition includes the Crocodilia in the Dinosauria, as it is absolutely con-, 
nected with the typical Dinosaurs by the Opisthoccela (Sauropoda Marsh). 
?Episternum present. 
Sn quite possible that the three divisions ot this head form one natural order, 
the Streptostylica, or Squamata. 
oe Including Ch oristodera, see AMERICAN NATURALIST, 1884, p. 815. 
