222 AN INTRODUCTION TO ZOOLOGY 



upgrowths of the basal plate, while in the otic regions they are com- 

 posed of the mesial walls of the auditory capsules. In the orbital 

 regions only a small part of the side wall is derived from upgrowths 

 of the trabecular plate ; it is mainly built up from two independent 

 dorsal elements, the orbital cartilages. These meet in the middle 

 line posteriorly, forming a cranial roof, and they also fuse with the 

 otic capsules. At the front end, however, they remain apart, 

 leaving between them the anterior cranial fontanelle. The rest of 

 the anterior end of the skull is formed by the cartilaginous nasal 

 capsules which are laid down around the olfactory vesicles. 



Visceral Skeleton. 



In Scyllium there is also connected with the skull a visceral 

 skeleton composed of seven pairs of arches one behind the other, 

 each composed of two half hoops of cartilage forming a support 

 for the anterior end of the alimentary canal. None of these are 

 actually fused with the cranium, although the first two are firmly 

 attached thereto by means of ligaments. The first arch is the 

 mandibular, composed of upper and lower jaws. The upper jaw or 

 palato-pterygo-Quadrate cartilage, is a stout curved bar of cartilage 

 somewhat compressed laterally and situated in the upper margin 

 of the mouth. Its anterior end curves inwards to the middle line 

 to meet its fellow in a ligamentous mandibular symphysis, and it is 

 attached to the cranium just behind the olfactory capsule by the 

 ethmo-palatine ligament. The posterior end is markedly flattened 

 and bears a strong ridge for the attachment of powerful jaw muscles, 

 and it further bears at its postero- ventral corner a surface for articu- 

 lation with the lower jaw. The lower jaw itself is similarly composed 

 of two flattened curved bars, Meckel's cartilages, lying in the lower 

 margin of the mouth, and they also meet in an anterior median 

 ligamentous symphysis. The cartilage is narrow in front, but much 

 deeper behind for the insertion of muscles, and here it not only 

 articulates with the upper jaw, but the two jaws are connected with 

 the hyomandibular cartilage. It is noteworthy then that neither 

 jaw is directly joined to the cranium, but only indirectly through the 

 intermediation of ligaments and the upper portion of the second or 

 hyoid arch. Such a skull is termed Hyostylic, in order to distinguish 

 it from an Autostylic one such as our own, in which the jaws are 

 directly connected with the cranium. It will be seen later that the 

 mandibular arch is much modified from a typical arch, and both parts 

 of it are covered by an integument bearing numerous rows of sharp 

 close-set teeth which are homologous with dermal denticles. The 

 folds of skin at the side of the mouth are supported by a pair of 



