12 Bullettfi American Museum of Natural History. [Vol. XIX, 



The scapula (Figs. 4, 5, sc.) is applied to the inner side of 

 the cleithrum, while the precoracoid, p. cor., appears to be 

 applied to the inner side of the scapula. There may, how- 

 ever, be some distortion here. In Tarpon the precoracoid is 

 applied to the inside of the cleithrum in front of the scapula, 

 but it also articulates with the latter. 



On the united scapula and coracoid are borne the surfaces 

 for articulation with the fin and its supports. Nearest the 

 cleithrum there is a rounded head (Fig. 4), which fits into 

 a cavity at the base of the fin. Beyond this are two surfaces 

 for the paired baseosts which Cope mentions; and still fur- 

 ther out is a row of six grooves for the reception of six un- 

 paired baseosts. Between this shoulder girdle and that of 

 Tarpon I see no important differences, except in the presence 

 of the paired baseosts. Of these the outer one corresponds, 

 no doubt, to the articulatory surface for the first baseost 

 of Tarpon. For the other, situated below the rounded head 

 on the scapula and mesiad of the surfaces for the other 

 baseosts, I find no equivalent in Tarpon. Possibly we must 

 credit to Protosphyrccna the possession of eight baseosts. 

 Of these the first has possibly been crowded out of its place 

 to a position below the rounded head on the scapula, 

 while the second has been crowded to a position above the 

 head. 



The disposition of the paired baseosts with reference to the 

 rounded head and the row of unpaired baseosts must limit 

 greatly the movements of the fin. In fishes there is generally 

 a free movement of the fin at right angles to its plane ; but it 

 seems that in Protosphyrcena there could have been only very 

 restricted motion perpendicular to the plane of the fin. The 

 action of the paired baseosts would have had the eftect of 

 steadying the motion in the plane. Such motion would have 

 had as its end the employment of the fin as a weapon, with 

 which its possessor could slash an enemy or a victim of its 

 appetite. The position of the paired and the unpaired base- 

 osts may be determined from Cope's figures (Vert. Cret. Form. 

 West, pi. liv, fig. 9), from the figures already cited of the 

 present paper, and from Fig. 6. In the latter figure the 



