OF THE SKFLL IN SHAEKS AND SKATES. 221 



branchial rays proceeds from each ; these are pedate at their outer ends. The ventral 

 segment (hypobranchial) is more or less adze-shaped (fig. 5, h.br) ; but this obtains 

 only in the second to the fourth. The first has this segment very long, at first dilated, 

 and then very slender, and the right and left are early fused together. The last 

 branchial arch has its pharyngo-branchial united to that of the fourth, and its hypo- 

 branchials {h.lr. 5) completely united. These fused elements look like an azygous 

 piece ; but in the adult of B. clavata (PI. XLII. fig. 4, h.hr 5) they are very partially 

 united, and, as to form, are seen to be only a modification of the adze-shaped or fan- 

 shaped type. 



At this stage the branchial arches, including the hyoid, carry two sets of branchiae 

 in full function. The external are at their highest development ; and the internal plates 

 are perfect, although small. 



The " extrabranchials " are absent, as far as I can make out, in difiierent kinds of 

 Skates' (i?. clavata, B. maculata, &c.) ; nor is there a labial on the mandible. But there 

 are four pairs of prseoral labials — three acting as nasal valves {I 2, 3, 4), and the pair 

 {1 1) attached to the side of the " rostrum," but not riding upon the nasal sac, as in the 

 Dog-fish. 



Third Stage : Emlryos of the Thornback Skate (Eaia clavata) nearly ready for exclusion 



from the egg-i^ouch. 



In the last stage the metamorphosis v^as complete ; this third stage is given for the 

 sake of the vertical and transverse sections, which reveal the architecture of this kind of 

 chondrocranium. A longitudinally vertical section with the brain removed (PI. XLI. 

 fig. 1) shows a hollow barge-like structure, with a cartilaginous bottom perfect, and 

 projecting as a free prow — the basitrabecular rostrum {b.tr) — whilst the " deck " is only 

 cartilaginous fore and aft over the ethmoidal and the auditory region. The various 

 nerve-outlets (1, 2, 5, 7, 8, 9, 10) are easily recognized. This little boat is undergirded 

 by transverse bars that appear in the section — the pterygoid, mandibular, and fii'st and 

 fifth hypobranchials (§'.J>5', mn, h.hr 1, h.lr 5). 



The first transverse section shows the nasal caps or domes {ol) far apart ; these are 

 connected by a large bridge of cartilage, convex below and concave above. Thus there 

 is formed a large praecranial space ; for the frontal skin is convex. 



An inner labial {1 2) is seen in section ; the palatal skin follows the convexity of the car- 

 tilaginous bridge ; that bridge is formed by the trabeculse and their commissure (tr.cm). 

 Here are the very elements of the nasal septum and roof of the higher types ; but the 

 trabeculte have only united below ; they are far apart ; and their crest applies itself 

 normally to the inner edge of the nasal dome, yet forms a structure widely difierent 

 from that which obtains when these domes are closely adpressed, and the trabecular 



' In the Torpedo the dilated ends of the hranchial raya unite outside the pouches in such a manner as to form 

 a practical " estrabranchial " band (Gegenbaur, pi. 13. fig. 3, pi. 20. fig. 1). 



VOL. X. — PART IV. No. 5. — March 1st, 1878, 2i 



