168 ZOOLOGY SECT. 



of the endoskeleton are supported by dermal fibre-like structures 

 (ceratotrichia) 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 structure 

 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 Kays 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 occipital 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 vestibuli 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. Some- 

 times 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. 823, hy. mn. ; Fig. 846, 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. 847) there is in addition to this a prominent post- 

 orbital process of the palatoquadrate for articulation with the 

 post-orbital region of the skull (amphistylic 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. 846, 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 Trygonorliina it articu- 

 lates with the dorsal portion of the first branchial arch. In the 



