1910.] The Skeleton of (he Monotrcmes: Anuly.sis of Charaetern. 155 



the tibia, and a shallow antero-internal depression for the internal malleolus 

 of the tibia. These characters are unic(ue among mammals. 



Analysis of skeletal characters. The characters reviewed above may be 

 classified under the following headings: (1) reptilian characters not jire- 

 served in higher mammals; (2) reptilian characters preserved also in higher 

 mammals; (3) characters of tloubtful origin; (4) primitive mammalian 

 characters not definitely known to be of reptilian heritage; (5) specialized or 

 aberrant characters. 



1. Reptilian characters not preserved in higher mammals: Squamoso- 

 parietal canal; cochlea auris not spirally wound; cervical vertebrae with 

 true ribs; scapula, with acromial border anterior, prespinous fossa rudimen- 

 tary; large coracoids and procoracoids, the latter overlapping in the middle 

 line; T-shaped interclavicle; glenoid fossa of shoulder girdle very near mid 

 line; long axis of humerus held in a nearly horizontal plane, elbows held 

 well out from the body; humerus with head elongate anteroposteriorly; 

 femur {OrnifJiorhj/nchus) flattened, head not sharply inclined to shaft, 

 greater and lesser trochanters very large, subequal; pelvis with small ob- 

 turator foramen. 



2. Reptilian characters, preserved also in varying degree in higher mam- 

 mals. Septomaxillary retained in embryo Echidna (p. 150); dumb-bell 

 bone (prevomers) of Ornithorlujnchus {cf. foetal Insectivora) ; mastoid large, 

 possibly homologous with opisthotic of reptiles; frontals not covering cere- 

 brum; very large pterygoids (ef. Edentata); occipital condyles with con- 

 siderable basi-occipital portion; stapes columelliform (cf. Manis); malleus 

 in ontogeny connected with ^Meckel's cartilage; 2r)-27 presacral vertebrse 

 (cf. 27 in Oudenodon, Broom, 1901, p. 27); humerus with stout ento- and 

 ectocondyles, entepicondylar foramen and globular capitellum for radius; 

 phalangeal formula, 2. 3. 3. 3. 3; elements of carpus and tarsus derived from 

 the plan preserved in mammal-like reptiles (pp. 440, 442); manus and pes 

 fully plantigrade. 



3. Characters possibly reptilian hut doubtful. Separate "post frontals," 

 developing in the chondrocranium ; entocarotid piercing floor of cranium; 

 optic foramen united with for. lac. anterius; zygomatic portion of squamosal 

 extending very far back, the glenoid fossa in Ornithorhynchus even over- 

 lapping the mastoid and lying external to the mastoid process. Ribs articu- 

 lating only with centra; ungual phalanges resembling those in Oudenodon. 



■ ■ 4. Primitive m,ammalian characters, not definitely known to be of 

 reptilian heritage. Chondrocranium unusually solid and massive; frontals 

 facing forward and upward; median parietal probably homologous with 

 paired interparietals, mastoid exposed on postero lateral part of brain case 

 and extending inferiorly into prominent mastoid process; tympanic ring 



