298 MR PHILIP J. WHITE ON THE 



side, namely, an epi- and a cerato-branchial segment, the former of which has its upper 

 extremity fused with the pharyngo-branchial of the fourth arch. 



The pharyngo-branchials. — These are somewhat flattened rods, having their inner 

 extremities free, and directed backwards and inwards (PI. I. Fig. 1, P&1-4), while their 

 outer extremities are thickened, and are connected with the upper ends of the epi- 

 branchials (E1-E5). The upper surfaces of the pharyngo-branchials present grooves 

 (PI. II. Fig. 2, B(/3) on which, in the recent state, the efferent branchial vessels rest. 

 In the first three arches the grooves have a direction from before, backwards and inwards, 

 while the grooves on the fourth pair of pharyngo-branchials have an almost transverse 

 direction from without inwards. The position of these grooves varies in the different 

 pairs of pharyngo-branchials. On the first pair the grooves lie over the outer extremities 

 of the cartilages, but in the case of the other arches they become more internal in position 

 and gradually deeper from before backwards. In Squatina there is a farther modification, 

 because here, instead of grooves, we find canals perforating the pharyngo-branchials of the 

 second, third, and fourth arches. 



Each pharyngo-branchial of the fourth arch possesses a process (PI. II. Fig. 3, P5) 

 which, springing from about the middle of its hinder border, and having a direction 

 outwards and slightly downwards, is continuous with the epi-branchial of the fifth 

 arch. This process is regarded by some as the pharyngo-branchial of the fifth 

 branchial arch. This process is absent in Hexanchus, and in Heptanchus it is only 

 slightly developed. Gegenbaur concludes from this that the process is developed from the 

 pharyngo-branchial of the fourth arch, in order that a point of attachment may be given 

 to the epi-branchial of the last arch, and that, therefore, it is not the representative of a 

 pharyngo-branchial. In Lsemargus, as I have already mentioned, this process and 

 the epi-branchial of the fifth arch are fused together, a condition which occurs in other 

 Elasmobranchs. 



The epi-branchials (PI. II. Figs. 2 and 3, E1-E5). — There are five pairs of these, 

 and from the first to the fourth they diminish in size. The fifth epi-branchial of either 

 side (E5), together with the pharyngo-branchial processes (P5) of the fourth arch, forms 

 an elongated rod. The upper extremities of the four anterior epi-branchials are movably 

 connected with the outer extremities of the pharyngo-branchials, while the fifth, as 

 already seen, forms a cartilaginous union with the fifth pharyngo-branchial. The lower 

 extremities of all are movably connected with the upper ends of the cerato-branchials. 

 Processes jut forwards from the upper extremities of the epi-branchials, and fossae, which 

 increase in depth from behind forwards, are found on the inner surfaces of the four 

 anterior cartilages. 



The cerato-branchials (PI. II. Figs. 2 and 3, Krl-Kr5). — These are the longest seg- 

 ments in the branchial arches, and have their upper ends connected with the epi- 

 branchials, but their lower extremities have various connections. The lower extremity of 

 the cerato-branchial of the first gill arch, has lying in front of it, the small hypo-branchial 

 (Fig. 3, Hi) belonging to this arch, and it also touches the basal portion of the 



