CETOTHERES FROM THE MIOCENE CHOPTANK FORMATION 



15 



sixth rib extends farther inward on the ventral surface. 

 The horizontally truncated neural spine rises 78 mm. 

 above the roof of the neural canal. Protuberant postero- 

 external facets for the capitulum of the seventh rib are 

 developed on both sides of the centrum. 



Seventh dorsal. — This dorsal (USNM 23794; pi. 8, 

 fig. 6) has a large nodular bony excrescence projecting 

 forward from the ventral border of the centrum on 

 the right side, which in conjunction with the backward 

 projecting excrescence of the preceding dorsal increased 

 the intervertebral space. An increase in the length (76 

 mm.) of the centrum, a reduction in the distance (214 

 nun.) between the ends of the diapophyses (pi. 6, fig. 6) 

 and a marked increase in the height (100 mm.) of the 

 neural spine above the roof of the neural canal are 

 the most obvious differential characteristics. The dorso- 

 ventrally compressed diapophysis pi'ojects horizontally 

 outward from the stout pedicle of the neural arch and 

 extends forward beyond the level of the anterior face of 

 the centrum. At the extremity of the diapophysis the 

 facet for the tuberculum of the seventh rib is divided 

 liorizontally into two articular surfaces. The prezygapo- 

 physial facets (pi. 7, fig. 6) are narrowed and separated 

 anterioi'ly by an interval of 80 mm. The postzygapo- 

 physial facets are narrower than those of the preceding 

 dorsal. No visible posteroexternal facet for the capitu- 

 lum of the eighth rib is developed on the left side; a 

 malformed protuberance is present on the right side. 

 The height (18 mm.) is less than one third of the width 

 (67.5 mm.) of the neural canal. 



Eighth dorsal. — Malformation (pi. 6, fig. 7), pos- 

 sibly the result in part of downward pressure, has 

 shortened the right pedicle of the neural arch and 

 depressed the right diapophysis, so that this process 

 is directed outward and obliquely downward. The left 

 diapophysis is bent upward toward its extremity. Both 

 diapophyses (USNM 23794) are dorsoventrally com- 

 pressed and widened at the extremity (pi. 7, fig. 7), on 

 which the facet for tlie tuberculum of the eighth rib is 

 located mainly on the ventral surface. Each low 

 metapophysis is thickened, projects forward beyond 

 the anterobasal edge of the diapophysis, and contributes 

 the external limit to the narrow prezygapophysial facet. 

 The prezygapophysial facets are separated anteriorly 

 by an interval of 47 mm. Postzygapophysial facets are 

 not developed. A somewhat wider neural spine (pi. 8, 

 fig. 7) rises 104 mm. above the roof of the neural canal. 

 No posteroexternal facets for the capitulum of the ninth 

 rib are present. A similar, but somewhat smaller bony 

 excrescence projects forward from the anterolateral 

 border of the centrum. Viewed from in front the profile 

 of the centrum is elliptical, but flattened dorsally. The 



neural canal is low (height, 22 mm.), but is reduced in 

 width (52 mm.). On the right side of the centrum a 

 wide backward projecting excrescence is located below 

 the usual position of the posteroexternal facet for the 

 capitulum of the following rib. 



Ninth dorsal. — The broad, dorsoventrally com- 

 pressed and bent upward parapophysis projects outward 

 solely from the dorsoexternal surface of the centrum 

 (USNM 23794). The thin (right, 18 mm.) anteropos- 

 teriorly widened (83 mm.) extremity of this process is 

 deeply excavated on its ventral surface for attachment 

 of the, head of the ninth rib. Tlie thick metapophyses 

 (pi. 7, fig. 8) are separated anteriorly by an interval of 

 38 mm., project forward beyond the le\el of the anterior 

 face of the centrum, and contribute the outer limit of 

 the short, narrow, and concave prezygapophysial facets. 

 The broad, backwardly inclined, and distally truncated 

 neural spine (pi. 8, fig. 8) rises 120 mm. above the roof 

 of the neural canal; the transverse diameter (49 mm.) 

 exceeds the vertical diameter (33 mm.) of the neural 

 canal. The elliptical profiles of the anterior (pi. 6, fig. 8) 

 and posterior ends of the centrum are quite similar as 

 are also their vertical and transverse diameters. A large 

 laterally compressed bony excrescence projects forward 

 from the ant«roexternal edge on the left side of the 

 centrum. On the posterior half of the right side of 

 the centrum a nodular excrescence projects laterally b\it 

 is not in contact with the low elongate malformation 

 bordering the anterior epiphysis. 



Tenth dorsal. — On the riglit side of the centrum 

 (pi. 8, fig. 9) of this dorsal (USNM 23794) a large later- 

 ally compressed bony excrescence projects forward from 

 the anteroexternal edge and another of similar size 

 extends backward from the posteroexternal edge. Tlie 

 left side of the centrum is normal. Ivonger parapophyses, 

 narrower neural canal (43 mm.), and longer centrum 

 conform to the usual sequential modification of ceto- 

 there dorsal vertebrae. Each parapophysis (pi. 7, fig. 9) 

 projects almost horizontally outward from the upper 

 portion of the external surface of the centrum and in- 

 creases in width toward its e.xtremity. On the extremity 

 (right, lengtli 94 nun.) a deep concave facet for the 

 head of the tenth rib occupies the posterior half, largely 

 on the ventral surface. The thin [)edicles fright, mini- 

 mum length, 63 nun.) of the neural arch extend two- 

 thirds of the length (99 mm.) of the dorsal surface 

 of the centrum. Tlie profile of tlie anterior face of the 

 centrum (pi. 6, fig. 9) approaches subcordate, flattened 

 dorsally. Tlie backward .slanting neural spine rises 125 

 mm. above the roof of the neural canal ; the width (43 

 nun.) exceeds the height (29 mm.) of the neural canal. 

 Thickened metapophyses project forward Iwyond the 



