SCAPULAR ARCH VERTEBRA. XV11 



The three anterior branchial arches are each composed of four pieces of 

 bone, which commencing from their inferior attachment are known as the' 

 hypobranchial, ceratobranchial, the epibranchial. In the fourth arch the 

 epibranchial is wanting, and superiorly the more expanded upper piece 

 which generally bears teeth, is known as the superior pharyngeal bone: 

 while the fifth arch is composed of the cerato-branchial only, and termed 

 the inferior pharyngeal. 



The opercular pieces or gill-covers have already been referred to, the 

 most anterior or innermost articulating with the tympano-mandibular arch 

 of the skull. In some fishes as sharks, rays, and cyclostomes, no gill- 

 covers are present, as will be subsequently remarked upon. 



The scapular arch which supports the pectoral fin is mostly joined to 

 the occipital bone, and according to Owen and Kitchen Parker, contains 

 the following bones commencing from above : the supra- scapular (post- 

 temporal), articulating with which is the scapular (supra-clavicular), and 

 attached to which is the coracoid (clavicular), while it is united below either 

 by suture or by ligament to the same bone on the opposite side. To this 

 last bone are attached two others, the radius and ulna (coracoid and 

 scapular), and two rows of small bones placed between the forearms and the 

 fin, or the carpals and metacarpals. Attached to the clavicular is a two- 

 jointed bone, the post-clavicular (epicoracoid). 



The ventral fins are attached to a pair of triangular dagger-shaped bones, 

 the pubic. 



Among the teleosteans there are numerous deviations from the percoid 

 and gadoid forms to which I have principally adverted, many of which will 

 be referred to in the following pages. 



Among the elasmobranchs are to.be found examples in which the noto- 

 cord may be observed without any trace of transverse segmentation up to those 

 in which there are distinctly ossified vertebras. As examples of persistent 

 notocords may. be mentioned among the plagiostomes the six-gilled shark 

 (vol. ii, page 308), and the spinous shark (vol. ii, page 323) ; while among 

 the Holocephala the arctic chimosra (vol. . ii, page 286) commences to 

 develop rings in the sheath of the notocord. In the majority of sharks, as 

 the blue shark (vol. ii, page 389), the vertebras have become completely 

 divided one from another, the individual bones being amphiccelous, and a 

 cavity existing .through the centre of the body of each thus permitting an 

 unbroken continuity of the remains of the notocord. In some forms many 

 of the anterior vertebras coalesce together, while the cranium is more or 

 less in one piece. 



Still further modifications occur in the cyclostomes (vol. ii, page 356), 

 wherein the notocord is not segmentated, but neural arches are represented 

 . by cartilages along either side of the spinal cord. In the lowest form or 

 Leptocardii (vol. ii, page 360) the type is exceedingly primitive. 



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