ANATOMY OF VERTEBRATES. 79 



The Cestracion, so interesting from its early introduction into 

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

 velopement as is the more modern Squatina. In the existing 

 species of the Australian seas (Cestracion Phillipi), 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 between the nasal capsules, which 

 meet at the middle line above the symphysis of the upper jaw. 

 The proper cranial cartilage is thinner than in the Squatina ; 

 the anterior or pineal fontanelle 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 backward, 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 support a series of massive grinding 

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

 jaw are confluent at the symphysis. 



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

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

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

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

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

 forms a long pyramidal rostrum, to which are usually articulated 

 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 under 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 cavities, which 

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

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



