CETOTHERES FROM THE MIOCENE CHOPTANK FORMATION 



19 



is present on the ventral surface of the centrum ; a sim- 

 ilar longitudhial ridge, however, is not developed on 

 the floor of the neural canal. 



CAUDAL VERTEBKAE 



Thirteen caudal vertebrae (USNM 23794), lying con- 

 secutively in a row, that have their epiphyses firmly 

 ankylosed to the centrum, were associated with this 

 skeleton. At least one small terminal caudal is missing. 

 The lengths of the three anterior caudals are approxi- 

 mately equal ; the length of the centrum diminislies more 

 abruptly behind the sixth caudal. The terminal caudals 

 are more noticeably compressed anteroposteriorly. 



The distance between the dorsoexternal edges of the 

 metapophyses progressively diminishes from the first 

 to the sixth. Behind the second caudal the height of the 

 neural spine rapidly decreases. Shortening of the trans- 

 verse process terminates in the short flange-like thick- 

 ened condition present on the sixth caudal. This trans- 

 verse process is perforated at the base medially on the 

 fourth, fifth, and sixth caudals- A vertical aqueduct on 

 the lateral portion of the centrum of the seventh, eighth, 

 and ninth caudals serves the same function for passage 

 of the segmental blood vessels on tlieir upward course. 



On the first caudal a short remnant of the ventral keel 

 on the posteriormost lumbar is retained in the median 



ventral longitudinal haemal groove; posterior, but no 

 anterior, haemal tubercles are present. Anterior and 

 posterior pairs of haemal tubercles, separated by the 

 median ventral longitudinal haemal groove, are present 

 on the third, fourth, fifth, and sixth caudals. On each 

 side of the centriun of the seventh and eighth caudal, 

 the anterior and posterior haemal tubercle is connected 

 by a thickened longitudinal bony connection which is 

 perforated medially by a foramen for passage of the 

 segmental blood vessels. Behind the eighth caudal, the 

 lateral portion of the centrum is pierced vertically by 

 an aequeduct for passage of these blood vessels. The 

 seventh may be regarded as the transitional caudal since 

 it has a reduced but complete roof for the neural arch 

 over the neural canal. 



The total length of this consecutive series of thirteen 

 caudals, including the cartilaginous intervertebral disks, 

 did not exceed 1270 mm. (50 inches). See table 14 for 

 measurements of the caudal vertebrae. 



First caudal.— On this vertebra (USNM 23794) a 

 pair of ventral posterior haemal tubercles contribute the 

 lateral limits of the median longitudinal haemal depres- 

 sion which rather unusually is divided at the middle of 

 its length by a longitudinal ridge. No anterior haemal 

 tubercles are present. The broad neural spine (pi. 11, 

 fig. 12) is broken off distally, but is inclined backward. 

 Both metapophyses are missing. The thin pedicles of the 



Table 14. — Measurements {in mm.) of the caudal vertebrae, USNM Z3794 



including haemapophyses. 



