Skull of long-beaked echidna (genus Zaglossus)
Family Tachyglossidae
The brains of all echidnas almost completely fill the skull, so the sulci on the cortical surface are matched by ridges on the interior of the skull that are clearly visible in CT scans (see Fig. of skull CT).
Average endocranial volume (ECV) for long-beaked echidnas (mean + SD) is 44.5 + 7.4 ml for Zaglossus bruijnii, 48.8 +13.1 ml for Zaglossus bartoni bartoni, and 50.4 + 8.1 ml for Zaglossus bartoni diamondi. Since the brain almost completely fills the skull and leaves faithful impressions on the skull interior, these values for ECV can be taken as good proxies for brain size (Ashwell et al., 2014).
Skull modified from Cabrera (1919)
References
Ashwell KW, Hardman CD, Musser A (2014) Brain and behaviour of living and extinct echidnas. Zoology 117, 349-361.
Cabrera, A (1919) Genera mammalium Madrid, 1919. biodiversitylibrary.org/page/32474795
Figure. CT scan of the skull of Zaglossus bartoni bartoni, with major features labelled. a) interior of right half; b) inferior view of skull vault; c) superior view of skull base.