SELACHII 135 



The Elasmobranchs are distinguished by the possession of a 

 heterocercal tail, ventral nostrils only incompletely subdivided by a 

 flap, a single series of cartilaginous branchial rays on each arch, 

 ampullary organs, a brain in which the front wall becomes much 

 thickened, a copulatory clasper in the male, an oviducal gland in 

 the female which secretes a horny egg-capsule. 



The sharply divided orders, Selachii and Holocephali, into 

 which the Elasmobranchii are at the present clay classified, appear 

 to have been already well marked in Devonian times. The 

 modern Holocephali are highly specialised in many respects, but 

 possess all the essential characteristics of Elasmobranch structure. 

 On the other hand, they differ fundamentally from the Dipnoi, 

 with which they have sometimes been associated on account of 

 a vague resemblance between the two groups in the skull and 

 vertebral column. 



Order 1. SELACHII. 



In this order, which includes the most primitive of living fish, 

 the spiracle, and the hemibranch on its front wall, are generally 

 preserved. The branchial slits always open independently to the 

 exterior, and are placed primitively in front of the pectoral girdle. 



The cartilaginous brain-case is large and very complete, except- 

 ing for an anterior dorsal fontanelle (Fig. 93). The two orbits 

 often communicate by a canal through the floor in the pituitary 

 region. The optic capsule may be connected with the cranium by 

 a cartilaginous peduncle. The intracranial notochord is usually 

 much reduced and a definite occipital joint becomes established 

 (Gegenbaur [153], Parker [318]). 



The notochord becomes much constricted by the mesoblastic 

 cells invading the sheath (p. 99), and vertebral centra are almost 

 always developed (Goette [167], Hasse [200], Klaatsch [265]). 

 These are usually strengthened by special calcifications, which 

 fii^st take the form of a constricted cylinder (Fig. 50) developed in 

 the inner layer near the notochord. There may be added radi- 

 ating longitudinal lamellae between the bases of the arches (Fig. 94), 

 or concentric cylinders outside the original one (Fig. 52). Hasse 

 called these three types the cyclospondylous, asterospondylous, and 

 tectospondylous respectively, and classified the Selachii accordingly 

 (Fig. 95). That these characters of the centrum are of consider- 

 able taxonomic value there can be no doubt, but the distinction 

 between the various types does not seem to be as clear and sharp 

 as was supposed. Both radiating and concentric calcifications may 

 be plainly shown in the same vertebra (Cetorhinus). The radiat- 

 ing calcifications may be developed centrifugally from the primary 

 cylinder (most Scyllioidei), or they may grow inwards centripetally 



