No. I.] 



THE GENUS OF FOSSIL FISHES. 



.47 



-X. thauinas. Two tail vertebrae. 



peculiarities which are of interest. I have a considerable 

 number of the vertebrae of X. thauinas, including unconnected 

 vertebrae belonging to the anterior portion of the column, and 

 a section about two feet in length from the tail region. I have 

 also numerous vertebrae belonging to some indeterminable 

 species, probably X. tnolossns. Access is permitted me to 

 vertebrae of probably X. molossus, belonging to the United 

 States National Museum. These belong mostly to the tail. 

 I will first describe the 

 peculiar structure of the 

 neural arches in the latter 

 region. The drawing pre- 

 sented will assist in the 

 understanding of my de- 

 scription (Fig. 12), The 

 neural arches here, as else- 

 where in this fish, are con- 

 nected with the centra 

 by suture, and have usually fallen out before burial, leaving 

 long grooves where their bases were inserted. This was the 

 case with the third vertebra behind the right-hand one 

 shown in Fig. 12. When we come to examine the arches 

 more closely we discover that each lateral half is not a single 

 piece, but consists of two pieces, a basal piece (a.n.a.) and the 

 arch proper {71. a.). That the proper arch is a distinct piece is 

 shown, not only by the existence of a suture, but likewise by 

 the fact that in the vertebra on the left hand of the figure the 

 arch has fallen out of its place before fossilization. The basal 

 or accessory piece is inserted by a shallow gomphosis into the 

 centrum for nearly the whole length of the latter. It rises 

 high in front and projects so far forward as to come into con- 

 tact with the basal piece of the next vertebra in front. Behind, 

 the basal piece is directed upward and backward in a rather 

 slender process, which abuts against the anterior edge of the 

 basal piece of the next vertebra behind. It is thus seen that 

 these basal pieces provide the anterior and posterior zygapoph- 

 yses. They remind us of the articulating processes of certain 

 other fishes, Mugil, etc. Between the anterior and posterior 



