THE SKELETON 57 
side of the median line from the adjacent and originally cartila- 
ginous supra-occipital, it may perhaps have existed in the ancestors 
of man as an independent bone.* 
The interparietals first appear in Mammals, but among the 
higher forms they are seen in a state of apparent degeneration, as 
would appear from their great variability in occurrence, form, and 
detailed relationships. They may, for example, remain either 
partly or wholly isolated; they may be either single, bilaterally 
symmetrical, or asymmetrical, or may be represented by but one 
lateral bone. 
Other inconstant ossific nuclei of this region are the prein- 
terparietalia. These may remain partly or wholly isolated, and 
show in form and position variations similar to those above 
described for the interparietals. The possible combinations of 
these anomalous bones cannot be discussed here (cf. Fig. 38). 
The morphology of the preinterparietals is not clear, and it 
is by no means unlikely that, like the ossa Wormiana (0. suturaria), 
they fall under the category of accessory ossicles. The problem is 
rendered still more difficult by the fact that, so far as is known, 
they are constantly present only in the Horses, while in other 
Mammals they are of mere sporadic occurrence. In Man, as 
compared with the latter, they appear comparatively frequently 
(1.e. 1 per cent). Equally uncertain is the morphology of the 
os fronto-parietale [os antiepilepticum of the ancients], a bone 
which occurs very rarely in Man, in the neighbourhood of the 
fronto-parietal suture. This bone, which is more often found in 
the Cebides among Monkeys, and less frequently in Rodents, may 
be sometimes paired. 
An atavistic significance may be probably attached to a 
bony process which occasionally appears in Man, behind and 
externally to the jugular foramen, and into which the rectus 
capitis lateralis muscle is inserted. This corresponds with the 
par-occipital or paramastoid processus of many Mammals, which 
attains its strongest development in Ungulates and Rodents. 
There is one more point worth consideration in the occipital 
region, we. the median portion of the linea nuche superior.’ 
A bony ridge (torus occipitalis), stretching at times as far as the 
linea nuchz suprema, occasionally develops here. According to 
1 Welcker regards all the larger bones which are occasionally intercalated in 
the lambdoidal suture as fragments of the os Incae. 
2 It is difficult to decide whether the furrow or pit (fossette vermienne, 
Albrecht), sometimes formed for the reception of the vermis cerebelli, has any 
phylogenetic significance. 
