74 



COMPARATIVE ANATOMY 



In Myxine, the extra-branchial basket-work is quite rudimentary and 

 amongst other peculiarities, the long nasal passage is surrounded by 

 cartilaginous rings, and communicates with the pharynx by a naso-palatine 

 duct. 



No fossil Cyclostomes are known, but Palceospondylns gunni from the Old 

 Red Sandstone of Caithness possibly shows affinities with this group. 



In the Elasmobranchii and Holocephali the skull presents 

 the simplest conditions and most easily comprehensible relations, 

 so that it may be taken as the starting-point for the study of the 

 skull of all other Vertebrates. It consists of a simple carti- 

 laginous and fibrous capsule either more or less immovably united 

 with the vertebral column (Squalida?,) or connected with it by 

 articulations only (Raiidse and Holocephali). 



ejc.br 



Ay.cn, 



FIG. 55. SKULL OF DOGFISH (Scyllium canicula}. (From T. J. Parker's 

 Biology, after W. K. Parker.) 



Cr. cranium ; aud.cp. auditory capsule; or. orbit; olf.cp. olfactory capsule ; r. 

 rostral cartilage ; hy.m. hyomandibular ; up.j. palatoquadrate ; l.j. Meckel's 

 cartilage ; hy. en. ventral part of hyoid arch ; Ig. Iff. ligaments supporting the 

 jaws from the cranium; lb. labial cartilage; br.a. 1 5, branchial arches; 

 br.r, br.r', branchial rays arising from the hyoid and branchial arches ; ex. 

 br. extra-branchial cartilages ; Nv. 2, optic foramen ; Nv. 5, foramen for 

 trigeminal and facial nerves. (The spiracular cartilage is not indicated.) 



True bones are never developed, the cartilage being merely 

 calcified. In Elasmobranchs the palatoquadrate and lower jaw 

 are provided with numerous teeth, arranged in rows ; in the 

 Holocephali, the teeth have the form of strong and sharp-edged 

 plates. 



The nasal region is often elongated to form a long cut- water or 

 rostrum (intertrabecula), at the proximal end of which the olfactory 

 sacs are situated, their cavities being separated from the cranial 

 cavity by a fibrous membrane (''lamina cribrosa"). Behind them 

 are the deep orbital hollows, which are bounded posteriorly by the 



