SKELETON 



137 



FIG. 95. 



intercalary (interdorsal). In the caudal region there are generally 

 two centra and two sets of arches to each segment (marked by one 

 pair of myotomes and 

 of spinal nerves) ; this 

 diplospondyly probably 

 ensures greater flexibility 

 in the tail (Hasse [200], 

 Ridewood [361]). 



The dorsal ribs are 

 often well developed in 

 the horizontal septum ; 

 of separate pleural ribs 

 only traces are perhaps 

 found in the anterior 



Caudal region (p. 68). Diagrammatic transverse sections of vertebrae, to 



TfiA rlrkveal fine or A illustrate the, A, cyclospondylous, B, tectospondylous, 



> are and ,C, asterospondylous T condition. C, notochord ; /), 



always much COncen- central calcareous ring ; E, elastica externa ; N, neural, 



. , _,, 11, an( l H, haemal arch. (After Hasse, from Sedgwick's 



trated ; the skeleton may zoology.) 



be either far removed 



from the vertebral column '(Set/Ilium) or closely connected with it 



.(Raja, Fig. 49 and p. 105). 



The pectoral girdle consists of a cartilaginous bar with a well- 

 .developed dorsal scapular region and a ventral coracoid region. 

 Ventrally the two halves are either fused or joined together by 

 fibrous connective tissue. A varying number of diazonal nerves 

 pass through the girdle to the fin. 



The two halves of the pelvic girdle fuse together in the mid- 

 ventral line to form a transverse bar (Fig. 96), generally pierced by 

 nerves. The pelvic girdle, on the whole, appears to have undergone 

 reduction, and almost all trace of a dorsal iliac process has vanished 

 in the sharks. 



The hyomandibular is large. The jaws are movably articulated 

 to the cranium, the suspensorium being hyostylic, in all modern 

 forms except the Notidani (p. 96). 



The lateral-line system of sense-organs, on the head and trunk, 

 sinks below the skin into a tube, which remains in communication 

 with the exterior by short canals between the sense-organs 

 <Fig. 97) (Ewart and Mitchell [134], Garman [148]). 



A dorsal thick- walled coecum, the rectal gland (Fig. 78), is 

 present at the hind end of the intestine (Howes [220]). 



Except in Chlamydoselachus, the branchial arches of the Selachii, 

 like those of the Dipnoi, have two efferent arteries (Fig. 57), 

 and, further, the epibranchial arteries differ from those of other 

 Pisces in that they correspond to the slits and not to the bars, being 

 formed by the union of a posterior efferent vessel of one bar with 

 an anterior efferent vessel of the bar next behind (Fig. 71). 



