OSTEOLOGY. 293 



Great anteater (Myrmecophaga), the nasals have kept pace with the growth 

 of the snout and appear at the dorsal side of the nasal aperture. Apparently 

 in Ornithorhynchus these bones are not thus excluded by the premaxillaries 

 which m this animal form the sides and anterior boundary of the opening. 



The series of skulls of the Proechidna shows certain features of interest 

 in regard to the order in which the sutures close. In the youngest specimen 

 (j\I. C. Z. 7,009) all the chief sutures are discernible with little difficulty, and 

 the broad jugal bones are so sUghtly fused with the cranium that they came 

 entirely away in cleaning. The sutures of the occipital portion of the skull 

 are especially clear, and the small median interfrontal is distinctly outlined. 

 In a second specunen (M. C. Z. 7,010) of practically the same size, the premaxil- 

 laries are thoroughly fused in the dorsal midline, from the tips of the nasals 

 to the nasal aperture, and the nasals are fused along their median line of contact 

 though still separate laterally from the surrounding bones. The interfrontal 

 is also with great difficulty to be traced, and the anterior edge of the large inter- 

 parietal is begmning to coalesce medially with the frontal. At the occipital 

 region of the skull the condylar portion of the basioccipital has become fused 

 with that of the exoccipital though elsewhere the occipital sutures are broadly 

 open. The parietal sutures are disappearing and the jugal is merging into the 

 cranial wall. The other sutures are still traceable. A third skull (M. C. Z. 

 12,415) has attained adult size and shows an advance in ossification. In dorsal 

 aspect the only sutures visible are: — the lateral and dorsal sutures marking 

 off the exoccipitals, the lateral suture bounding the supraoccipital, the suture 

 between the jugal and the maxillary process of the zygomata, and the sutures 

 delimiting the maxUlaries from the frontals, nasals, and premaxillaries. The 

 nasals are soUdly fused together and to the premaxillaries, though these are 

 still distinct laterally and ventrally from the maxillaries. Conspicuous rough- 

 ened furrows are developed on the vertex of the cranimn for the attaclmient 

 of muscles, and the posterior expansions of the jugals are soUdly fused with the 

 skull. In ventral aspect, the posterolateral boundaries of the basioccipital 

 are cjuite obliterated but elsewhere the sutures delimitmg this bone are still 

 open, so that in appearance the basi- and exoccipitals are fused into a single 

 bone whose boundaries are everywhere distmct. The bones of the palatal area 

 are still unfused, but those of the orbit have become well soldered together. 



As in the Echidna the palatal branch of each premaxilla becomes much 

 draw^n out posteriorly and extends as a narrowly tapering process to the origin 

 of the zygomatic portion of the maxilla. This bone seems to increase slightly 



