76 LECTURE IV. 



The hyoid arch in the Sqtiatina, as in most other Plagiostomes, con- 

 sists of two long and strong lateral pieces or cerato-hyoids {cormia 

 of Anthroj)otomy), and a median flattened symmetrical piece, the basi- 

 hyoid, {corpus ossis hyoidei) below. Short cartilaginous rays extend 

 outwards from the back part of the cornua, supporting the outer mem- 

 branous wall of the branchial sac : these answer to the branchiostegal 

 rays in osseous fishes, and support the diverging appendage or limb 

 of the hyoidean arch. But the fold of integument in which they 

 project is not liberated, and is continuous with that supported by the 

 opercular rays from the tympanic pedicle. Five branchial arches 

 succeed the hyoidean ; but are suspended, as in the Lamprey, from 

 the sides of the anterior vertebrte of the trunk. 



The Cestracion, so interesting from its early introduction into the 

 seas of this planet, is not so far advanced in cranial development as is 

 the more modern Squatina. In the existing species of the Australian 

 seas {^Cestracion Phillipi, v. pi. 10.), the cartilaginous basi-occipital 

 retains a deejj conical excavation, adapted to a corresponding one in 

 the atlas, which cavity is consolidated by cartilage in the Squatina ; 

 the original j^lfice of the extended anterior end of the chorda, along 

 the middle of the postei-ior half of the basi-cranial cartilage, continues 

 membranous, and the pituitary 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 forwards, 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 forwards and outwards to support the 

 nasal sacs, and are continued backwards, 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 the 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 Bays 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 membranous for 

 a greater or less extent, except in Narcine, where it is closed by 



* The basi-occipital also affords a small but distinct intermediate surface between 

 the two large condyles in the Zygana. 



