402 STUDIES FOR STUDENTS 



undisputed position among the fishes. Including both the sharks 

 and rays, the group may be defined as made up of forms in which 

 the skeleton is cartilaginous, the skin filled with fine calcareous 

 particles (shagreen), the tail heterocercal, and the external open- 

 ings of the gills mere slits in the skin of the neck unprotected 

 by an operculum. In the previous sub-class there are no remains 

 preserved of distinct vertebrae, but in the sharks the beginning 

 of the vertebrae is seen in the formation of cartilaginous rings 

 in the sheath of the notochord. These rings are of varying 

 degrees of development in the different forms, in some forming 

 mere circular bands around the chord, while in others they are 

 nearly closed by the ingrowth of the cartilage that tends to 

 segment the chord off into intervertebral elements. There is 

 always attached to the superior and the inferior faces of the 

 cartilaginous ring the neural and the haemal' arches that carry 

 the spinal cord and the blood vessels. 



The most primitive of the fossil sharks comes from the 

 Lower Carboniferous of Ohio. This form, Cladoselache , is, in 

 many respects, quite close to the hypothetical type form of all 

 the fishes, the body is long and slender, and there were seven gill 

 slits in the neck, which seems to be the number characteristic of 

 the earliest forms. The unpaired fins have not progressed 

 beyond the second stage of development, as outlined in the 

 first part of this paper, that is, the fin fold is supported by small 

 rods of cartilage, radials, that are not attached to the body 

 wall. The paired fins are in a scarcely more developed condition, 

 the lateral fold has disappeared, but the two lappets that repre- 

 sent the pectoral and the ventral fins have not progressed 

 beyond the stage of the radial support, and are consequently 

 of no value to the fish as swimming organs, but must have 

 served merely as balancers. The tail was abruptly heterocercal. 

 The whole form was rather small, not reaching a length of more 

 than six feet at the outside. 



Acanthodes, a rather small form from the Coal Measures of 

 England, seems to present a step in advance of Cladoselache. 

 The shagreen particles that are scattered throughout the skin 



