404 HARRIS HAWTHORNE WILDER ON 
stretch from the outer end of the premaxillaries to the quadrates. The five angles of 
the pentagon may be named from the bones that compose them, (1) premanillary, (2 
and 3) two quadrate, and (4 and 5) two opisthotic ; and the five sides in a similar way 
may be termed (1) occipital, (2 and 3) the paraquadrate, and (4 and 5) the palato-ptery- 
void. When dissected or macerated sufficiently to show its parts, it gives the distinct and 
correct impression of a cartilaginous structure overlaid by flat dermal bones laid on and 
overlapping “like shingles,” to borrow Parker’s apt comparison, and showing in places 
certain intervals through which the primordial skull and its ossifications are distinctly 
seen. When viewed dorsally (pl. 63, fig. 2) the most conspicuous dermal elements are 
the frontals and the parietals, which form almost the entire roof, and at the anterior end 
of the former appear the little premaxillaries. Of the maxillary arch, which normally 
extends from the outer end of the premaxillary to the quadrate, there is no trace, other 
than a firm ligament when in the recent state, but the inner arch, parallel to this, is well 
represented by the vomer and the palato-pterygoid, the outer edges of which are seen in 
the figure. The only other dermal bones visible from above are the paraquadrates which 
are seen extending along the postero-lateral sides of the pentagon. The dorsal aspect dis- 
plays also parts of four cartilage bones: the quadrates forming the antero-lateral angles, 
the pro-otics and opisthotics in the interval between the parietals and paraquadrates, and 
the exoccipitals which border the foramen magnum and form the condyles. Upon the 
ventral side (pl. 63, fig. 3) the most conspicuous bone is the huge parabasal, a dermal bone 
nearly covering the roof of the mouth, anterior and lateral to which are seen the denti- 
gerous premaxillaries, vomers, and palato-pterygoids. The paraquadrates form a part of 
the outer margin and a conspicuous process from them is attached to a similar process that 
projects from a very small oval bone, the operculum, which closes the opening into the 
otic capsule. The same four cartilage bones seen from the dorsal side appear here, three 
of which strengthen the otic region while the fourth forms the suspensorium for the 
attachment of the mandible. 
The Chondrocranium. 
If, now, as may be easily accomplished in a macerated skull, the dermal or “ shingle” 
bones be carefully removed, there remains a very curious piece of cartilage suggestive of 
an inner framework, the primordial skull, or chondrocranium. The cartilage bones, pre- 
viously noticed and now entirely uncovered, are very evidently the result of localized 
processes of ossification within this, which have taken place in what was once a continuous 
cartilaginous mass, the homologue of the wholly cartilaginous skull of the present 
Selachians. 
