316 Mr. H. G. Seeley on Acanthopliolls platypus, 



pair ; they do not arise quite so far forward, but extend back, 

 widening and thickening almost to the posterior articular sur- 

 face ; they make the widest part of the centrum. Below these 

 ridges the sides of the centrum converge inferiorly to the 

 hypapophysial ridges ; between these limits the depth of the 

 side is 1^ inch ; above the middle of this area is a faint hori- 

 zontal ridge which divides it into two unequal parts and gives 

 it a convex aspect. The narrow under surface is limited by 

 the two faint hypapophysial ridges, which slightly approxi- 

 mate in the middle and diverge towards the two ends, termi- 

 nating posteriorly in the oblique facet which is confluent with 

 the posterior articulation. The posterior side is unequally 

 six-sided, in every case a long side having a short side oppo- 

 site to it, there being a long superior margin and a short in- 

 ferior margin, two short sides above and two long sides be- 

 low. Both articulations are rather conspicuously concave. 



In the third vertebra the centrum is equally long, but is 

 much smaller, the posterior articulation measuring more than 

 1^ inch from above downward, and nearly 2 inches from side 

 to side ; while in the second vertebra the similar surface mea- 

 sures If inch from above downward and 2^ inches from side 

 to side. In the third bone the first pair of ridges become 

 stronger, the second become much fainter, and the obscure 

 third ridge is now a well-marked tumid ridge : in consequence 

 of these modifications the lateral spaces of the sides become 

 more concave from above downward. The hypapophysial 

 ridges have approximated much closer together, and become 

 more elevated, especially in front, showing that the chevron 

 bone now articulates with both the vertebree between which it 

 is placed ; and there is a marked increase in the concavity of 

 this under surface from front to back. 



The fourth bone is badly preserved. 



In the fifth vertebra the length of the centrum is 2 inches ; 

 but the depth of the posterior articulation, including the 

 chevron surface, is -^ inch, while its width from side to side 

 is 1^ inch; the lateral surfaces are markedly concave; and the 

 whole bone looks like a substance contracted and withered. 

 The first and second pairs of lateral ridges have disappeared ; 

 and the third ridge is now a strong elevated ridge, dividing 

 the side into two equal parts, and at its terminations making 

 the widest part of the articular ends. The hypapophysial 

 ridges become parallel, rounder ; and tlie whole under surface 

 from back to front is deeply concave. The posterior articular 

 surface is only slightly larger than the anterior end ; and the 

 facets for the chevron bones are nearly equal. The inter- 

 vertebral cup is becoming less deep. 



In the sixth bone the centrum is If inch long. The side 



