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. In a 

 second miniature 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 magnimi to the median edge of the supraoccipital. In an adult 

 skull belonging to the United States national musemn, 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 smiilar conditions occur in the 

 skull of the Echidna. At one tune 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 selbststandiger Knochenkern in vorderen Theil 

 der sogenannten Parietalplatte des Primordialcranium." I have found what 

 seems to be a homologous bone in a number of specmiens of immature gophers 

 (Geomys). Its presence is due perhaps to some irregularities of ossification. 

 In the Proechidna it fills a smaU 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 uiterfrontal. This bone is quite diff'erent 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 



