220 



6th. In some peculiar cases (Squall, possessing a nictitating eye- 

 lid, viz. Mustelus, Carcharias, Galeus, Sphyrna) the cartilaginous 

 arches remain separated, and then the intermediate periosteal layer 

 performs the part of an osteogenic stratum. The osseous parts pro- 

 duced in this way lie at the same places as the bony plates mentioned 

 under 4 and 5 ; they always possess the form of wedges, and coalesce 

 with the chordal vertebral body, in some cases only at their ends, in 

 others in their whole length. Although these ossifications are not 

 developed from cartilage and have a very peculiar structure they 

 consist of a calcified fibro-cartilage with peculiar ossified strong fibres 

 running straight through their whole thickness, it is clear enough 

 that they exactly correspond to the above-mentioned plates and 

 wedges of other Plagiostomes formed out of the coalesced crural 

 cartilages. 



From certain modes of transformation of the sheath of the chorda, 

 combined with certain changes of the outer ossifying layer, the 

 following types in the composition of the vertebral bodies may be 

 established. 



TYPE I. The vertebral body takes its origin entirely from the 

 proper sheath of the chorda. 



A. Sheath of the chorda thick. 



1st. Vertebral bodies soft (fibro-cartilaginous), incompletely 

 separated from each other, and only distinguished by the interior 

 septa of the chorda. Hexanchus. 



2nd. Vertebral bodies partly cartilaginous, with annular ossifica- 

 tions of the form of short double cones. Ligamenta intervertebralia 

 very strong. Heptanchus (anterior vertebra). 



3rd. Vertebral bodies wholly cartilaginous, with thin osseous 

 double cones of good length in the middle of the cartilaginous body. 

 Centrophorus. 



4th. Vertebral bodies well ossified, cylindrical and strong, formed 

 inside by strong osseous double cones, and outside by alternating 

 layers of cartilage and bone. Sqiiatina. 



B. Sheath of the chorda thin. 



5th. Vertebral body a thin hollow osseous cylinder ; chorda pro- 

 per in its whole length cylindrical. Leptocephalus, Helmichthys, 

 Hyoprorus (last vertebra). 



