OSTEOLOGY. 289 
foramen magnum. In one specimen (M. C. Z. 12,415) the exoccipitals meet in 
the median line and wholly exclude the supraoccipital from the foramen. Ina 
second immature skull (M. C. Z. 7,398) this bone is likewise wholly dorsal to 
the exoccipitals, but the latter do not quite meet in the median line so that 
a deep and narrow emargination is left between them, extending dorsally from 
the foramen magnum to the median edge of the supraoccipital. In an adult 
skull belonging to the United States national museum, a similar condition 
seems to exist, for there is a narrow rounded emargination of the foramen at 
its median dorsal border. Although all sutures are obliterated, there can be 
no doubt that this emargination is due to the failure of the exoccipitals to meet 
ventral to the supraoccipital. In a fourth specimen (M. C. Z. 7,010) the last- 
named bone does reach the foramen magnum and forms its dorsal border 
between the exoccipitals, for a space of about 3.7 mm. Like variations are 
recorded by Weber (1904) in this animal and similar conditions occur in the 
skull of the Echidna. At one time these differences were even regarded as of 
taxonomic value. 
Abutting against the entire anterodorsal edge of the supraoccipital and 
extending forward nearly to the nasal region is a large median bone which is 
generally considered the homologue of the interparietal. It covers the greater 
portion of the dorsal part of the head and extends laterally nearly or quite to 
the dorsal margin of the large bone forming the posterior part of the zygomatic 
arch. It seems to be an unpaired bone, although cranium M. C. Z. 7,010 shows 
a trace of a small suture-like mark posteromedially. 
The frontals are rather small anteriorly and expand laterally to form the 
dorsal two thirds or more of the orbit. Between them, posteromedially, there 
appears in at least two specimens (M. C. Z., 7,009, 7,010) a small separate bone, 
of nearly oval outline. A similar bone seems to have been discovered by van 
Bemmelen in the Echidna ‘“‘als selbststaéndiger Knochenkern in vorderen Theil 
der sogenannten Parietalplatte des Primordialeranium.”’ I have found what 
seems to be a homologous bone in a number of specimens of immature gophers 
(Geomys). Its presence is due perhaps to some irregularities of ossification. 
In the Proechidna it fills a small space left where the frontals and the anterior 
emargination of the interparietal fail to come together. For lack of a better 
name I have called it an interfrontal. This bone is quite different from the 
so-called postfrontals of Sixta and Lubosch. The latter lie one at each side of 
the large interparietal directly posterior to the frontals, and help to form part 
of the lateral wall of the cranium. At their posterodorsal angle they meet 
