No. I.] 



THE GENUS OF FOSSIL FISHES. 



43 



cleithrum and coracoid belonging ventrad of the fin articulation 

 being mostly wanting. Fortunately I have on one block both 

 the right and left halves of the shoulder girdle in nearly perfect 

 condition. To one half are also attached some of the remark- 

 able fin rays of this genus. A figure is presented of the right 

 half of the girdle seen from without (Fig. 9). In this figure the 

 cleithrum conceals a part of the coracoid,^ but the latter is 

 so broad that a consider- 

 able portion of it is seen. 

 In Tarpon there is along 

 the upper border of the 

 coracoid a long fontanelle 

 between this bone and 

 the cleithrum. If such a 

 fontanelle was present in 

 Xiphactinus it is con- 

 cealed beneath the clei- 

 thrum. In Tarpon there 

 are two or three foramina 

 in the coracoid just below 

 the scapula. They are in 

 life closed by membrane. They are wanting in Xiphactinus. 

 The outer surface of the dorsal limb of the cleithrum of 

 Xiphactinus is broad and convex to the very hinder border. It 

 thus resembles Tarpon, and differs from Alosa sapidissivia, in 

 which the hinder portion of this surface is rough and excavated 

 for muscles. In the extinct genus there is an extensive fossa 

 on the inner surface of the upper limb of the cleithrum. The 

 upper half of this fossa lies between an outer and an inner 

 plate of the cleithrum. Further down, the fossa is limited 

 mesially by the precoracoid. There seems to be no such fossa 

 in Tarpon, and that of Alosa is very shallow. In both Tarpon 

 and Alosa the precoracoid is a much less important bone than 

 it is in Xiphactinus. 



Fig. q. —Xiphactinus. Shoulder girdle. 



1 I employ for the elements of the shoulder girdle the terms in common use, 

 except that I use Gegenbaur's name cleithrum instead of clavicle. For the latter 

 element Dr. Gill has proposed the term proscapula; for coracoid, hypocoracoid ; 

 for scapula, hypercoracoid ; and for precoracoid, mesocoracoid. 



