BY WILLIAM A. HASWELL, M.A., I3.SC. 105 



marking the position of the posterior semi-circular canal, which 

 is absent in Torpedo, is well-developed in the Australian genus. 

 Neither the prse-orbital nor the post-orbital processes are 



developed, and the orbit is devoid of basal plate. There is 

 neither ethmoidal notch nor foramen. 



The olfactory region resembles that of Torpedo but there 

 are no distinct praeorbital processes, and the fins articulate with 

 the anterior extremities of the nasal capsules. 



The hyo-mandibular is a long cartilage, longer than the greatest 

 width of the skull, dorso-ventrally compressed and broad proximally, 

 but narrowing distally ; it is divided into two by a distinct 

 transverse suture (as in many Kays) the outer piece being much 

 shorter than the inner. Both upper and lower jaws are extremely 

 slender. The hyoid arch is small, and quite similar to, though 

 smaller than the branchial arches ; above it is incomplete, and not 

 in any way connected either with the skull or with the hyo- 

 mandibular, its dorsal end articulating with the basal of the first 

 branchial arch, which articulates with the postero-external process 

 of the skull in the manner already noticed. Though this arrange, 

 ment of the anterior visceral arches is not, as will subsequently be 

 seen, peculiar to the present form, it is a moditication which has 

 hitherto escaped notice. In the Sharks the hyoid arch proper 

 (which is much more important and much more sharply marked 

 off from the branchial arches than in the Rays) is suspended from 

 the skull by means of the hyo-mandibular, with which it articulates 

 towards its distal extremity. The hyo-mandibular cartilage is 

 thus in the Selachoidei a true hyo-mandibular suspensorium. 

 But in the majority of the Rays, as pointed out by Gegenbaur, it has 

 no such dual function, the hyoid arch being but slightly, if at all, 

 connected with it, and its sole function being that of a mandibular 

 suspensorium. In Torpedo the basal of the hyoid arch unites 

 with the postero-lateral angle of the skull ; it is free below except 

 where a short cross-piece unites it with the ventral end of the 

 first branchial arch. The first branchial arch is attached to the 

 ventral end of the second, and the second and third have each 

 distinct copular pieces uniting with the basilar plate. 



