ANATOJIY OF VEETEBEATES. 79 



The Cestracion, so interesting from its early introduction into 

 the seas of this planet, is not so far advanced in cranial dc- 

 vclopcment as is the more modern Squatina. In the existing 

 species of the Australian seas (^Cestracion Phillijn), the cartilagi- 

 nous basioccipital retains a deep conical excavation, adapted to a 

 corresponding one in the atlas, which cavity is consolidated by 

 cartilage in the Squatina ; the original place of the extended ante- 

 rior end of the chorda, along the middle of the posterior half of 

 the basicranial cartilage, continues membranous, and the pitui- 

 tary perforation is permanently closed by membrane only ; the 

 basal cartilage expands anterior to this, and comes into close 

 connection with the maxillary arch, and is thence continued 

 forward, contracting to a point ))etween the nasal capsules, which 

 meet at the middle line above the symphysis of the up])er jaw. 

 The proper cranial cartilage is thinner than in the Squatina ; 

 the anterior or pineal fontancUe forms an extended membranous 

 tract on the upper part of the cranium ; the vertical ridges, which 

 rise from the sides of this tract, extend forward and outward to 

 support the nasal sacs, and are continued laackward, interrupted 

 by a notch filled by membrane, to the posterior angular processes, 

 which overhang the joint of the maxillo-hyoidean pedicle. The 

 maxillary and mandibular arches are as simple as in Squatina, 

 but much stronger, since they sujiport a series of massive grinding 

 teeth, as well as pointed ones, or laniaries. The rami of the lower 

 jaw are confluent at the symphj^sis. 



The Skates and Eays have the skull movably articulated, as in 

 Squatina, by tAVO basilar condyles and an intervening space, to 

 the axis. The skidl is flat and broad ; the upper wall mem- 

 branous for a greater or less extent, fig. 64, except in Narciii.e, 

 where it is closed by cartilage. The anterior or vomerine part 

 forms a long pyramidal rostrum, to which arc usually artieidatcd 

 cartilages connecting its extremities with the anterior angles of 

 the enormously developed pectoral fin, ib. 12 : in the space 

 between the skull and those fins, the Torpedo carries its electric 

 batteries. The tympanic pedicles, are short and thick ; the 

 maxillary and mandibular arches long and wide, stretching trans- 

 versely across the imder part of the head. 



In the ordinary Sharks the forward prolongation of the cranial 

 cavity gives a quite anterior position, and almost vertical plane, 

 to the fontanelle : three columnar rostral cartilages are produced, 

 two from above, and one from between the nasal canities, which 

 processes converge and coalesce to form the framework of a kind (if 

 cut-water, at the fore-part of the skull. In the place of articular 



