1906.] SELACHIAN FISHES. 735 



especially in the case of genera of doubtful position, I have endea- 

 voured to confirm or to extend the observations which have been 

 made by others. The endoskeleton of the Selachii may be conveni- 

 ently considered under the heads: (1) the axial skeleton, or cranium 

 and vertebral column ; (2) the visceral skeleton, comprising the 

 labial cartilages, jaws, and branchial arches ; and (3) the pterygial 

 skeleton, comprising the supports of the median and paired fins, 

 including the pectoral and pelvic arches and the mixopterygia. 

 Modifications in structure of these three systems are considered 

 in the account which follows. 



In all living Selachians the vertebral column is made up of 

 the notochord and its cartilaginous sheath and of dorsal and 

 ventral series of paired cartilages attached to the lattei'. These 

 paired cartilages consist of principal pieces, neural plates (basi- 

 dorsals) and hsemal plates (basi-ventrals), and of intercalary pieces 

 alternating with these, the interneural and interh^emal plates. 

 Centra may be formed by the segmentation of the cartilaginous 

 notochordal sheath. The neural plates are typically broad at the 

 base and narrowed above and vertebral in position, whilst the 

 interneurals are correspondingly nai'rowed below and intervertebral 

 in position. Both neurals and interneurals may meet in the 

 middle line and unite above the spinal cord ; or if the interneurals 

 of one side are juxtaposed above the apices of the intervening 

 neurals, then only the interneurals may so unite. 



A median series of cartilages may sometimes apjDarently be 

 segmented off from the united paired elements. The neurals and 

 interneurals may not meet above, and in that case the roof of the 

 neural canal may be completed by a longitudinal strip of cartilage, 

 apparently of independent origin, usually, but not always, seg- 

 mented*. Sometimes the incomplete union of neurals and inter- 

 neurals leaves a series of interspaces, which are filled by a median 

 series of cartilages f. The plates composing the neural and 

 haemal arches may each become secondarily segmented into two or 

 more pieces +. 



The heemal plates correspond to the neurals and are vertebral 

 in position, whilst the interhasmals are intervertebral. The latter 

 are often reduced or absent, especially in the caudal region. In 

 the praecaudal region the hsemals remain separate and may bear- 

 rib-like cartilages, which are intermuscular in position and probably 

 not homologous with the ribs of Teleostomi. In the caudal region 

 the hfemals unite below to form a complete hsemal arch, and a 

 median series of cartilages may be segmented off. Primitively the 

 neural and haemal plates are loosely attached to the chordal sheath, 

 but sometimes they become more intimately united with it, and 



* In aj'oung CarcJiariasmelanoptemsl find an unsegmented median longitudinal 

 rod of cartilage completing the neural canal. 



t The median cartilages of Scyliorhinus are probably not derived from the 

 neural arches. 



X Compare Hasse's plates of various Hypotremata and also Helbiug's figures of 

 Lcemca'gus horealis and L. rostratus. 



Proc. Zool. Soc— 1 906, No. XLIX. 49 



