1910.] Genetic Relations of the Mdnotrcmfs: Sitmuuiry. 159 



It' the two genera were extinct and known only from imperfect skulls 

 their close relationship might be doubted or unsuspected. 



The more significant ])oi)jfs of agrremrnf between the two famili(\s include 

 the following: 



(1) The fundamental similarities (combined with differences in propor- 

 tion) in the scapula, coracoid and procoracoid, humerus, radius and ulna, 

 carpals, pelvis, femur, tibia and fibula, tarsals, and tarsal spur. The 

 astragalus and calcaneum also present a unicjue ordinal type (p. 154). 



(2) The underlying similarity in the cranium {e. g., chondrocranium 

 massive, rounded, large median ])arietals, very small frontals, backward 

 prolongation of palate, pterygoids and palatines entering into floor of brain 

 case, jugal vestigial or absent, arrangement of foramina, etc.). 



(3) The close similarity in the reproductive system. In this instance, 

 as in many others, the characters of the girdles, limb-bones, carpals aiul 

 tarsals, and reproductive system are more significant of affinity (pp. 110, 

 111) between adaptively divergent animals than the characters directly 

 relating to food habits, such as the dentition, jaws, digestive tract, etc. 



Summary of the Genetic Relations of the Monotremes. 



The Monotremes resemble the Anomodonts (proper) in the phalangeal 

 formula and in many characters of the tarsus, humerus and shoulder girdle; 

 they retain structures in the embryo which have been interpreted as the last 

 vestiges of scales (p. 145) and the reproductive organs of both sexes exhibit 

 numerous reptilian characters; so that one is at first tempted to adopt the 

 hypothesis that they have been derived from some family of Permian or 

 Triassic reptiles other than that (or those) which gave rise to the higher 

 mammals. Upon this hypothesis the numerous characters, which give 

 the Monotremes their systematic rank as mammals would have to be ex- 

 plained as parallel develoj:»ments; and if this were so it would seem rea- 

 sonable to assume that the mammal-like reptiles which gave rise to the 

 Monotremes had already begun to acquire the first rudiments of hair, of 

 the diaphragm of lacteal glands etc. But as to this, there is some indirect 

 evidence that not even Cynognathus, which was surely more advanced 

 toward the mammals than w^ere the Anomodonts, had proceeded very far 

 toward acquiring the distinctly mammalian characters above mentioned. 

 The hard palate of Cynognaifivs was limited to the anterior end of the 

 pharynx. Here then is evidence of a reptilian mode of respiration and 

 consequently of a relatively unstable body-temperature. But it has already 

 been shown (p. 147) that the development of both a hairy covering and of 



