72 THE ELASMOBRANCPI FISHES 



The trunk vertebrae lie between the pectoral and pelvic regions and their 

 dorsal and ventral plates are usually distinct and regular. In some types, 

 however, as for example Laemargus horealis, the dorsal intercalary plates 

 represent the maximum of change in that each plate is subdivided into two or 

 more parts. A similar segmentation in the ventral intercalary plates may be 

 present in this area. 



The anterior vertebrae of this region are rib-bearing but there is great 

 variation in the number of ribs present. In Laemargus horealis ribs are present 

 on only a few vertebrae, while in other forms ribs may extend almost to the 





_i ^ ..' J J , ■< / ' ' 



.■■■ ^ ^ .r -* -■ 



»^"««?«ii;*<ss^-3*3^^^;^^ 



\ 



B 



Fig. 78. Caudal vertebrae. 



A. Heterodontus francisci. (Duncan Dunning, del.) 

 'B. Bldnohatis productus. (Chester Stock, orig.) 

 h.s., haemal spine; id., dorsal intercalary plate. 



posterior limits of the trunk area. The posterior part of the trunk segment 

 offers great variation in the different elements of the column and is of particu- 

 lar interest because of diplospondyly or doubling of the segments, which is 

 present here. 



Diplospondyly may begin immediately after the last rib-bearing centrum 

 (Heterodontus, fig. 77b), or a series of vertebrae may intervene before the 

 diplospondjdous vertebrae are reached. In Scyllium, according to Ridewood 

 (1899), a brief area of transition follows the rib-bearing segments in which 

 the stages from monospondyly to diplospondyly may be traced. The first indi- 

 cation of the change is seen in the slight shifting backward of the dorsal 

 intercalary plate of the first vertebra behind the last one bearing a rib, so that 

 one of the neurals rests directly upon the dorsal basal plate. In the vertebra 

 following, this condition is further accentuated and results in diplospondyly, 

 which is also seen in succeeding vertebrae. In a type like Rhinohatis (fig. 77a) 

 the rib-bearing segments (r.) extend far posteriorly and are separated from 

 the true diplospondylous segments by only a few vertebrae. In this type, also, 

 the spines are of large size and rest between two or more of the irregular 

 intercalary pieces. 



