xni PHYLUM CHORDATA 173 



part, of expanded plates of cartilage. The marginal portions of the 

 unpaired fins beyond the limits of the endoskeleton are supported 

 by dermal fibre-like structures (ceratotrichici), composed of elastin. 



The skull is an undivided mass of cartilage, hardened, in many 

 cases, by deposition of calcareous matter, but not containing any 

 true bony tissue. It consists of a cartilaginous case for the pro- 

 tection of the brain and the organs of special sense. The struc- 

 ture of this cartilaginous brain-case as it occurs in the Dog-fish has 

 already been described. The main differences observable in the 

 different families are connected with the size and form of the 

 rostrum. In the Rays the lower lip of the foramen magnum is 

 deeply excavated for the reception of a short process, the so-called 

 odontoid process, which projects forwards from the anterior vertebral 

 plate, and on either side of this is an articular surface the occi- 

 pital condyle for articulation with corresponding surfaces on that 

 plate. In the Sharks the skull is not so definitely marked off 

 from the spinal column. The apertures of the aqueductus vesti- 

 buli in the Rays are not situated in a median depression such 

 as is observable in the Dog-fish and in all the Sharks. The 

 articular surface in the auditory region for the hyomandibular is 

 sometimes borne on a projecting process, sometimes on the 

 general level of the lateral surface. Sometimes in the Rays 

 there is a smaller articulation behind for the first branchial arch. 



The upper and lower jaws the palatoquadrate and MeckeVs car- 

 tilage are connected with the skull through the intermediation 

 of a hyomandibular cartilage (Fig. 813, hy. mn. ; Fig. 835, h. m.). 

 The skull is thus of the hyostylic type as regards the mode of 

 suspension of the jaws. In the Sharks the palatoquadrate has 

 a process (absent in the Rays) for articulation with the base of 

 the skull in the pre-orbital region. In Hexanchus and Heptan- 

 chus (Fig. 836) there is in addition to this a prominent post- 

 orbital process of the palatoquadrate for articulation with the 

 postorbital region of the skull (ampTiistylic arrangement). Ces- 

 tracion is also in a sense amphistylic ; the palatoquadrate is 

 firmly united with the skull, articulating with a groove on the 

 base, and the hyomandibular takes only a small share in the 

 suspension of the jaws. At the sides of the mouth in all Elasmo- 

 branchs are a series of labial cartilages, usually two pairs above 

 and one pair below. Attached to the hyomandibular is a thin 

 plate of cartilage the spiracular (Fig. 835, sp.) which supports 

 the anterior wall of the spiracle. 



The hyoid arch proper is in most of the Elasmobranchs con- 

 nected at its dorsal end with the hyomandibular sometimes 

 at its distal extremity, sometimes near its articulation with the 

 skull ; but in some Rays it is not so related, but articulates 

 separately and independently with the skull behind the hyo- 

 mandibular, and in the genera Hypnos and Trygonorhina it articu- 



