220 MR. W. K. PAEKEE ON THE STEUCTUEE AND DEVELOPMENT 



(the quadrate, q), has grown forwards, and has become a large upper jaw, the " pterygo- 

 quadrate ;" whilst the remainder of the primary stem hinges with the outturned end of 

 this as the lower jaw— articulo-Meckelian (PI. XL. figs. 4 & 5, q.pg, ar.imi). 



These two arches [two by metamorphosisj in rest are but slightly bowed forwards, 

 where they meet their fellow bars of the opposite side ; they are almost directly trans- 

 verse, and conform, as every thing else does, to the flat outspread form of this peculiar 

 type of Fish. 



The second postoral (hyoid) was also forked above in the early embryo ; the further 

 development of that arch is similar to what takes place in the Teleostean {e.g. Salmon): 

 but it is arrested at a lower morphological level. 



The anterior fork of the second postoral articulates with the auditory capsule beneath 

 the pterotic ridge (" tegmen tympani") by an oblong condyle above (PI. XL. fig. 4, h.m), 

 whilst below it becomes detached entirely from the rest of the bar, turns forwards close 

 behind the spiracle, which it protects, as the metapterygoid does in front, and then 

 fastens itself strongly by ligamentous fibres to the quadrate, becoming its new additional 

 suspensorium. 



The hinder fork, or proper apex, is wholly freed from the hyo-mandibular ; its upper 

 piece, or " epihyal," is in reality only half the epihyal region, having lost the hyo- 

 mandibvilar wedge. Thus the expanded apex of the archis cleft obliquely, as in Saurop- 

 sida and Mammalia, and not from top to bottom, as in Teleostei, nor simply across 

 without subdivision of the " epihyal " region, as we have just seen in the Dog-fish. The 

 bar, freed from the anterior fork, is now developed into a rather feeble branchial arch ; 

 it is attached by ligament to the end of the jutting pterotic above ; and the upper or 

 epihyal segment is exactly like the epibranchials (PI. XL. fig. 4, e.liy, e.br), save that it 

 is smaller than most of them, and, being attached to the corner of the skull, sends 

 no " pharyngo-pleural " segment over the pharynx. There is no secondary cartilage 

 developed in the attaching ligament, as in Teleostei ; the " interhyal " (" stylohyal," 

 Cuv.) is fibrous. 



The ceratohyal (chi/) is feeble, but normally branchial in character; the arch is 

 finished below by a small styliform hypohyal segment ; this is attached by ligament to 

 the first hypobranchial (fig. 5, Ji.Ji!/, Ji.br 1 ). 



Here we see a vast difference between the Shark and Skate ; for in the former the stout 

 two-membered bars of the hyoid have their ventral ends strongly articulated to a large 

 basihyal piece ; whereas in the Skate the lower ends of the hyoid are nearly as far 

 apart as the breadth of the transverse mouth. 



The five branchial arches (br 1-5) are very uniform, only decreasing gently in size from 

 before backwards. Each is composed of a superpharyngeal, apical piece, the pharyngo- 

 brancbial, an epibrauchial, a ceratobranchial, and a hypobranchial on each side (jj.br, 

 e.br, c.br, h.br). These arches are strongly bowed outwards, and bent on themselves; 

 their lateral parts are thick, and grooved externally. A single series of cartilaginous 



