1910.] Genetic Relations of the Monotremes: Summary. 161 



reproductive system, shoulder girdle and pelvis, but also they have had time 

 to acquire so many deep seated peculiarities in the skull that the Monotreme 

 skull may be set in a class by itself, in contrast to the primitive Marsupio- 

 Placental type. 



The immediate common ancestor of Ornithorhyuchus and Echidna may 

 provisionally be conceived as follows: 



In the present cycle of life habits both existing types of Monotremes are 

 animalivorous, and hence it may be inferred that the common ancestor was 

 probably insectivorous. The true teeth of Ornithorhynchus indicate that in 

 the stem form the cheek teeth were short crowned and irregularly cuspidate, 

 perhaps analogous to those of Myrmecohius, but few in number. Front 

 teeth were lacking. The snout was rather short, tapering and somewhat 

 depressed at the end (cf. embryonic Monotremes); the orbito-temporal 

 fossa was short, the maxillary and sciuamosal portions of the zygoma were 

 stout, the jugal (as in Placental Insectivores) was already reduced. The 

 glenoid fossa of the squamosal was located well back, but not so far as in the 

 existing genera, the angle of the jaw was inflected {cf. Ornithorhynchus) and 

 the pterygoid muscles well developed. The hard palate, as in Phlangers, 

 Edentates and Insectivores, was carried well backward, probably not fene- 

 strated (cf. p. 220), and the pterygoids were large and flattened; the ethmo- 

 turbinal complex was well developed, the frontals were small, sloped down- 

 ward and forward and did not cover the cerebrum ; the median parietal was 

 large, the occiput rounded, the occipital plane slightly inclined forward, 

 the mastoid exposure large; the broad condyles as in Armadillos may have 

 been correlated with the habit of digging with the snout; the foramen mag- 

 num was surmounted by a median vertical opening. 



The large odontoid was suturally separate from the axis and ended in a 

 cylindrical peg which articulated with the basioccipital. The six cervical 

 ribs were well separated from the centra and the vertebral artery passed 

 between the head and the tubercle of each; in the dorso-lumbar vertebrae 

 the spinal nerves notched the base of the neviral arches and the epiphyses 

 were reduced. The dorsolumbars were of nearly uniform size and character 

 with low neural spines, the neural arches being broad and flat in top view, 

 with prominent prezygapophyses, which permitted but little lateral move- 

 ment. The lumbar parapophyses were reduced or absent. There were 

 about 17 dorsals and perhaps three or four lumbars (p. 152). The primary 

 sacrum consisted of only one vertebra; the caudal parapophyses were broad 

 and flat. 



The majority of the foregoing characters indicate semi-fossorial and 

 perhaps partly amphibious habits; and it is also a fair inference from the 

 existing genera that in the ancestral form the body was stocky, the ribs. 



