SHARKS AS ANCESTRAL FISHES. 



25 1 



position is suggested in the adjoining figure (Fig. 4, B). The shoulder 

 girdle appears to be represented in a pair of flattened " coracoid " 

 elements. 



The unpaired fins seem, as shown in a specimen in the Museum 

 at Delaware, O., to have been outlined as in Fig. 1. Two dorsals are 

 present, spineless, their shape corresponding somewhat to the ventrals, 

 the foremost one slightly the larger. The latter includes the 



FIG. r 4 



4. Pectoral fin. 



5. Ventral fin. 



6. Caudal fin. 



B. Basal cartilages. 

 D. Dermal membrane. 

 N. Neural spines. 

 R. Cartilaginous fin-rays. 



Figs. 4-6.— Fins of Cladoselache. 



supporting elements of about fifteen metameres, arranged similarly to 

 the radials of the paired fins ; their basal supports have not been 

 determined. 



The caudal fin is unique in its structure (Fig. 6). It is certainly 

 of the elasmobranchian type, heterocercal, but its widely upturned tip 

 has brought it to the limits of homocercy. Yet, although it has 

 become specialised for its swift-swimming function, it seems evident 

 that this fin is nevertheless of an exceedingly primitive type, more 



