32 HAV. [Vol. II. 



basioccipital. They arise about opposite the union of the basi- 

 sphenoid and the parasphenoid. These strong lateral processes 

 are almost wholly absent in Tarpon. In both this genus (Fig. i, 

 pa.s.) and Xiphactinus there is, on each side, a strong process 

 arising from the parasphenoid to meet the prootic. These 

 processes form the side walls of the muscular canal. This canal 

 was of greater extent perpendicularly in Xiphactinus than in 

 Tarpon. 



The basisphenoid is a Y-shaped bone, the upper end of which 

 articulates with the prootics, while the lower end rests on the 

 parasphenoid. It is almost twice as long as the corresponding 

 bone in a tarpon of the same size (Fig. i, b.s.). 



So far as can be determined from the crushed skulls of 

 Xiphactinus, the form and relationships of the alisphenoids and 

 the orbitosphenoids were very much the same as in Tarpon. In 

 this latter fish both of these pairs of bones are large (Fig. i, 

 als., O.S.). The alisphenoids meet in the mid-line, below the 

 brain, and thus continue forward the floor of the brain-case. In 

 front of these are the large orbitosphenoids, ankylosed in the 

 mid-line, as in the salmon. There is no distinct presphenoid. 



In the tarpon the brain-case is roofed over behind by the 

 supraoccipital. In front of this the prootic sends upward and 

 inward a plate of bone which meets a similar plate from the 

 opposite prootic. This roof is continued forward by plates of 

 bone which grow mesiad from both the alisphenoids and the 

 orbitosphenoids. These two pairs of bones also send out great 

 lateral plates, which abut against the postfrontal and the lower 

 surface of the frontal. In the mid-line above the brain, the 

 united orbitosphenoids send upward a more or less interrupted 

 crest of bone. Between the brain-case, as thus roofed over, and 

 the parietals, pterotic, and frontals, there is a great space an 

 inch high and extending from one side of the skull to the other, 

 and in life this is probably filled with the primitive cartilage. 

 The arrangement of this portion of the head may be understood 

 by an examination of Parker's figures of the salmon {Trans. 

 Phil. Soc. London, vol. clxiii, pp. 95-145, Pis. I-VIII). In 

 Tarpon there are two foramina in the pro5tic which open from 

 the outside into the cavity here described. One of these is 



