294 ZAGLOSSUS. 
in length up to the time when the premaxillaries fuse dorsally, after which 
there is practically no change in its length. The continued elongation of the 
rostrum seems thereafter to take place by growth at its basal portion so that 
the termination of the premaxillary is carried progressively farther forward 
from the ventral margin of the orbit. This is in accord with the fact that the 
sutures at the base of the rostrum are the last to close in the dorsal view of the 
skull. Thus in a partly grown skull M. C. Z. 7,010, in which the premaxillaries 
have fused dorsally, their extreme ventral length is 73 mm., and from the pos- 
terior tip to the orbit is 12.6 mm. In the fully grown skull, M. C. Z. 12,415, 
in which these sutures are still visible, the extreme length of the premaxillary 
branch is 74 mm., practically as in M. C. Z. 7,010, but from its posterior tip to 
the orbit is 29 mm., indicating that later growth has been entirely at this basal 
portion of the rostrum. The premaxilla at the same time has been carried 
forward and a line is left in posterior continuation of the cleft into which its 
tip fits. 
Cranial Measurements of the Proechidna. 
Type | 12,415 | 12,414 | 22,992 | 7,010 | 7,398 
Greatest length = 172 178 182 143 | .146 
Basal length —_— 161 167 169 134 | 136 
Palatal length (from posterior process) _— 145 152 156 120 | 122 
Tip of rostrum to orbit ventrally 108 | 103 109 113 85 84 
Least interorbital width 19 19 20 19 20 20 
Greatest width of brain-case 56 55 55 55 54 52 
Anterior margin of orbit to temporal canal _— 45.5 | 44.5 41 43 41.5 
Greatest length of mandible = 130 138 139 111 | 110 
Two quite adult skulls have also been studied. These are of the size of 
M. C. Z. 12,415 but show a stage farther in ossification. The first (12,414) is 
in the collection of the Museum of Comparative Zodlogy; the second (22,992) 
is in the collection of the United States national museum. In both, the sutures 
are totally obliterated, and the crania are solid masses of bone, except that the 
tympanic rings (wanting in 22,992) have not fused with the rest of the skull. 
Rugosities are developed for muscle attachment on the vertex of the skull, 
and just back of the orbit a slight ridge or process is developed on the frontal 
which may be considered a postorbital process homologous with that of other 
mammals. A small swelling opposite it near the anterior end of the jugal 
defines the orhit, back of which the broad plate-like expansion of the jugal 
covers over nearly all of the temporal fossa. The rami of the lower jaw though 
