EXPLANATION OF PLATE A. 



Fig. 1. Basal half of dorsal fin-spine of Chimseroid Fish, side view, with outline 

 cross -sections of lower (la) and upper (Ib) ends, and front view of piece 

 of anterior keel (Ic), nat. size. Portland Stone, Portland. [Collection 

 of F. J. Barnes, Esq.] 



Fig. 2. Ischyodus sp., side view of portion of dorsal fin-spine, nat. size. Portland 

 Stone, Portland. [British Museum, No. P. 6036.1 





