404 HARRIS HAWTHORNE WILDER OX 



stretch from tlic outer end of the premaxillaries to the quadrates. The five angles of 

 the pentagon niav he named from the bones that compose them, (1) premaxillary, (2 

 and 3) two quadrate, and (4 and 5) two opisthotic ; and tlic five sides in a simihar way 

 may be termed (1) occipital, (2 and 3) the paraqiiadrate, and (4 and 5) the palato-ptery- 

 goid. When thssected 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 tlistinctly 

 seen. When viewed dorsally (pi. Go, fig. 2) the most conspicuous dermal elements are 

 the frontals and the parietals, which form almost tlie 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 (pi. 63, fig. 3) the most conspicuous Ijone 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-pterygoid s. The paraquadrates form a part of 

 the outer margin and a consjiicuous 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 suspensorimn 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- 

 \nousl_y 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. 



