There remain certain other membrane bones which are often found connected with the cranium. Of these, one of the largest is the _parasphenoid_ which, in Ichthyopsids, is found underlying the basicranial axis. _Prefrontals_ often, as in most reptiles, occur lying partly at the sides and partly in front of the frontal, and _postfrontals_ similarly occur behind the orbit lying partly behind the frontals and partly at their sides. Lastly a _squamosal bone_ is, as in Mammals, very commonly developed, and lies external and partly dorsal to the auditory capsules. THE JAWS AND VISCERAL SKELETON. In the most primitive fish these consist of a series of cartilaginous rings or arches placed one behind another and encircling the anterior end of the alimentary canal. Originally they are mainly concerned with branchial respiration. The first or =maxillo-mandibular= arch forms the upper jaw and the lower jaw or mandible. The second or =hyoid= arch bears gills and often assists in attaching the jaws to the cranium. The remaining arches may bear gills, though the last is commonly without them. The above condition is only found in fishes, in higher animals the visceral skeleton is greatly reduced and modified. The first or maxillo-mandibular arch is divisible into a dorsal portion, the =palato-pterygo-quadrate bar=, which forms the primitive upper jaw and enters into very close relations with the cranium, and a ventral portion, =Meckel's cartilage=, which forms the primitive lower jaw. The cartilaginous rudiments of both these portions disappear to a greater or less extent and become partly ossified, partly replaced by or enveloped in membrane bone. The posterior part of the palato-pterygo-quadrate bar becomes ossified to form the =quadrate=, the anterior part to form the palatine and pterygoid, or the two latter may be formed partially or entirely of periosteal bone, developed round the cartilaginous bar. Two pairs of important membrane bones, the _premaxillae_ and _maxillae_ form the anterior part of the upper jaw, and behind the maxilla lies another membrane bone, the _jugal_ or _malar_, which is connected with the quadrate by a _quadratojugal_. The premaxillae have a large share in bounding the external nasal openings or anterior nares. In lower vertebrates the nasal passage leads directly into the front part of the mouth cavity and opens by the posterior nares. In some higher vertebrates, such as mammals and crocodiles, processes arise from the premaxillae and palatines, and sometimes from the pterygoids, which meet their fellows in the middle line and form the palate, shutting off the nasal passage from the mouth cavity and causing the posterior nares to open far back.