PHYLUMViii VERTEBRATA 141 



haemal arches, or their halves may unite in a single piece. Among existing 

 reptiles there are never more than two sacral vertebrae, but the sacrum of 

 üinosaurs sometimes comprises as many as ten, and that of Pterosaurs from 

 three to six. When more than three are present, however, they are usually 

 coössified into a single piece (synsacrum). Only among Ophidians, Pythono- 

 morphs, and Ichthyosaurs is there no differentiated sacral region. Small 

 cuneiform intercentra (hypocentra) may be inserted between the cervicals, 

 caudals, and a portion or all of the dorsals. 



All the vertebrae with the exception of the posterior caudals may bear 

 ribs. Cervical ribs are as a rule short, and have the distal end expanded in 

 hatchet-shaped fashion. When a sternum is present, the ribs with which it 

 is connected facilitate the distinction between cervical and dorsal regions. 

 The posterior dorsal ribs have free extremities, and lumbar vertebrae are 

 always non-costiferous. Cervical ribs are usually double-headed, and the 

 centra exhibit two facettes for their attachment. JDorsal ribs may be either 

 Single or double-headed, and in Chelonians are remarkable for their union 

 with the broad costal plates of the carapace. Many of the Reptilia develop 

 so-called abdominal ribs, which are ossifications of the connective tissue, and 

 correspond to the ventral scales of Stegocephalians. 



The shidl agrees more nearly with that of birds than amphibians in its 

 general features, and the ossification of the primordial cranium is much more 

 complete than in the latter group. The investing membrane bones are so 

 intimately united with true cartilage bones that a distinction is often 

 impossible. The basiocciput articulates with the atlas by means of a single, 

 sometimes tripartite condyle, formed either by the basioccipital alone, or by 

 this bone in conjunction with the exoccipitals. Both the basioccipital and 

 supraoccipital may be excluded from the borders of the foramen magnum. Of 

 the bones forming the auditory capsule, the proötic (petrosal) is situated in 

 front of the lateral parts of the occipital region ; it occurs as a separate bone, 

 and its front margin is pierced for the third brauch of the trigeminus nerve. 

 The opisthotic is often fused with the exoccipital, and the epiotic with the 

 supraoccipital. Besides the usual auditory opening, or fenestra ovalis, there is 

 commonly a fenestra rotunda ; the position of the former is either between the 

 exoccipitals, or between the opisthotic and proötic. At the base of the skull 

 immediately in front of the basioccipital occurs the basisphenoid, a true carti- 

 lage bone, in place of a parasphenoid ; and this is continued in front by the 

 presphenoid. 



Alisphenoids and orbitosphenoids are as a rule wanting, or are replaced 

 by downwardly directed processes of the parietals and frontals. The last- 

 named bones are usually of large size, and may be either paired or unpaired. 

 Applied to the parietals on either side is a large squamosal, which takes part 

 in the posterior cranial border and that of the supratemporal vacuity. The 

 lower end of the squamosal usually abuts against the quadrate, which is 

 always strongly developed, and bears a condyle for articulation with the lower 

 jaw. Sometimes the squamosal is divided, the upper portion being identified 

 as the prosquamosal or supratemporal. In Sphenodon the two components 

 later become coalesced. 



Among the Squamata the quadrate is movably articulated with the squa- 

 mosal, being united by ligaments only (Streptostylica) ; but among other 

 reptiles it is rigidly attached to the skull by sutures (Monimostylica). The 



