CEKVICAL AND DORSAL VERTEBRA OF POLYPTYCHODON. 435 



easily defined ones of a big head and the short cervical centrums 

 described. 



It is probable that several cervical vertebras are missing from the 

 series ; and although for convenience I shall number the vertebras 

 3rd, 4th, 5th, 6th, and 7th, these numbers must not be taken to indi- 

 cate the sequence of the vertebras in the neck, for several may be 

 missing between the axis and No. 3. 



The third vertebra has its anterior face in the form of a pentagon, 

 the median angle being in the middle of the base of the vertebra. 

 It is nearly 4 inches deep, and 41- in transverse diameter in the middle 

 of the vertebra. It is apparently much more cupped than the cor- 

 responding face of the axis. It measures rather more than 1 J inch 

 from back to front in the middle of the centrum ; at the base, as pre- 

 served, it measures 2-J- inches from back to front, while at the neural 

 canal the antero-posterior measurement is 1-2- inch. The neural canal, 

 as in the axis, is somewhat impressed ; it is not more than J inch 

 wide. On each side of the canal are the facets for the neural arch, 

 each more than an inch wide. Below these facets are the unarticular 

 sides of the centrum fully 1^ inch deep. Then, below the middle 

 of the vertebra succeed the impressed facets for the ribs ; they have 

 a diameter of 1§ inch, are more deeply impressed behind than in 

 front, and extend across nearly the whole antero-posterior length of 

 the vertebra. Below these surfaces is the external unarticular base 

 of the centrum; it is 3} inches long, terminates on each side in a 

 sharp ridge, and is prolonged forward in the middle as though a 

 subvertebral wedge-bone were blended with its anterior margin. 

 The posterior surface presents a corresponding concavity on its 

 basal border. These outlines are repeated in the succeeding neck- 

 vertebras. The posterior articular surface is more elliptical, smaller, 

 and flatter, rather more than 4 inches wide, and rather more than 

 3| inches deep. 



The fourth vertebra is badly preserved, but appears to have 

 resembled the third. 



The fifth vertebra increases a little in antero-posterior measurement ; 

 and the facet for the cervical rib becomes larger, and ascends a little 

 up the side of the centrum. The articular faces remain flattened ; 

 but there is now on each a low, flattened, broad, central elevation, 

 between which and the margin is a concave area. The back-to- 

 front measurement is everywhere about 2 inches, though at the 

 base it is a trifle less. The facets for the neurapophyses are placed 

 a little further forward, so as not to reach within | inch of the 

 posterior articulation ; each is an inch in diameter. The sides of 

 the centrum are worn. The articular facets for cervical ribs, 11- 

 inch in diameter, are so placed that their superior margin is above 

 the middle of the centrum. The base measures 4 inches in trans- 

 verse diameter, in which . direction it has become a little more con- 

 vex, while from front to back it has become rather less concave. 



In the sixth vertebra the larger articulation for the rib is so 

 placed that half is above and half below the middle of the centrum. 

 The width of the vertebra across these facets is 5 inches ; and though 



