1906.] SELACHIAN FISHES. 737 



The notochord may be unconstricted and its sheath unseg- 

 mented, as in the Holocephali and in the greater part of the ver- 

 tebral column of Chlmnydoselachus ; or it may be constricted by an 

 annular thickening of the cartilaginous sheath below the middle of 

 each neural' plate, but without the formation of a calcified ring and 

 with impei-fect segmentation, as in Ilexanchus ; or calcified rings 

 may be formed in the chordal sheath, which usually take the form 

 of double cones, so that the notochord is constricted vertebi'ally 

 and expanded intervertebrally, whilst the centra are Ijetter defined 

 and more completely separated. This type of centrum, with a 

 calcified double cone and without fui-ther calcification, as in the 

 Squalidse, is termed " cyclpspondylic " by Hasse (test-fig. 116, E). 

 Secondary calcification may be dejDOsited round the central double 

 cone either homogeneously or as a sei-ies of concentric lamellae, and 

 may form a complete and continuous investment, or may be broken 

 up by uncalcified areas so as to present a radiating pattern in cross 

 section. " Asterospondylic" centra (text-fig. 116, A, B & C) may be 

 defined as those in which the secondary calcification lea,ves four 

 principal uncalcified areas radiating from the central double cone 

 to the bases of the neural and hsemal arches, and are characteristic 

 of the suborder Galeoidei, although in the Scyliorhinidfe a series of 

 modifications set in which culminate in a complete reversion to 

 the cyclospondylic type in the geiiem jPristiurus and Fseudotriacis. 

 .,_ " Tectospondylic " centra (text-fig. 116, D & F) are those with 

 well-developed secondai-y calcifications not arranged on the astero- 

 spondylic plan. Hasse has applied this term to the various types 

 of centra found in the Batoidei, and in Sqiaitina and Pristiopliorus, 

 and it is impossible to give any definition which will include these 

 and exckide Cestracion. Probably also the so-called asterospondylic 

 centra of some Hybodonts would have to be included. 



In the prtecaudal region the vertebrse are as numerous as the 

 myotomes, and the neural plate is typically either perforated or 

 notched posteriorly for the exit of the ventral root of a spinal 

 nex-ve, whilst the interneiu^al is perforated or notched posteriorly 

 for the exit of the dorsal root. In the greater part of the caudal 

 region of all Plagiostomi the vertebrae are twice as numerous as 

 the myotomes, and consequently every other pair of neurals and 

 interneurals are not perforated or notched for nerve-exits. To 

 this condition the name " diplospondyly " has been applied, and 

 the condition which obtains in the caudal region of the Holocephali, 

 where the vertebrae, as ascertained by the number of neurals and 

 interneurals, are more than twice as numerous as the myotomes, 

 as indicated by the nerve-exits, has been turned " polyspondyly " 

 by Hasse. 



The transition from monospondylic to diplospondylic myotomes 

 may be abrupt, as in Bqualus, or there may be an intermediate 

 region. Thus in Eeptranchias, as described by Mayer, the dupli- 

 cation of the arches precedes that of the centra, a number of these 

 bearing two joairs of neurals and h^emals, so that every other pair 

 of interneurals in this region corresponds to the middle of a 



49* 



