CETOTHERES FROM THE MIOCENE CHOPTANK FORMATION 



21 



lateral portion of the centrum, and presumably reach 

 the posterior end of the open neural canal. The meta- 

 pophyses (pi. 12, fig. 8) are reduced to low, thickened 

 elongate knobs. The neural canal is an open elongated 

 groove. A reduced longitudinal ridge (pi. 14, fig. 7) 

 marks the position of the transverse process on the pre- 

 ceding caudals. 



Ninth caudal. — Absence of haemapophyses (USNM 

 23794; pi. 12, fig. 9) and a further shortening of the 

 centrum characterize this caudal (pi. 13, fig. 9). Except 

 for the elongated central orifice of the vertical aqueducts 

 of the segmental blood vessels, the ventral longitudinal 

 haemal groove is almost obliterated. One additional ori- 

 fice on each side completes the nonnal three ventral ori- 

 fices of the vertical aqueducts that pierce the centrum 

 and enable the segmental blood vessels to emerge dor- 

 sally through two foramina in the deep open neural 

 groove. On the small terminal caudals of Recent mysti- 

 cetes, the neural canal is not covered by the roof of the 

 neural arch and chevron bones are not attached; these 

 anteroposterior^ compressed caudals serve as a base 

 for the attachment of the fibrous caudal flukes. 



Tenth caudal. — A depressed but flattened anterior 

 end (pi. 12, fig. 10) and a convex posterior end of the 

 anteroposteriorly compressed and somewhat circular 

 centrum are the most obvious distinguishing features 

 of this caudal (USNM 23794). The dorsal pair of ori- 

 fices (pi. 13, fig. 10) for the vertical vascular aqueducts, 

 separated by an interval of 5 mm., open into a short 

 ovoidal cavity. Ventrally there are three smaller ori- 

 fices for the segmental blood vessels, each lateral one 

 being 24 mm. distant from the central orifice. 



Eleventh caudal. — The centnun of this caudal (pi. 

 12, fig. 11; USNM 23794) has become more quadrangu- 

 lar in outline, the anterior end is depressed, and the 

 posterior end convex. A rather broad longitudinal 

 groove is present on each lateral surface. Dorsally, the 

 two orifices of the vertical vascular aqueducts (pi. 13, 

 fig. 11) are closely approximated in a small (width, 

 7 mm.) circular cavity. The usual three small ventral 

 openings are pre.sent. 



Twelfth caudal. — Viewed from in front, the profile 

 of this caudal (USNM 23794; pi. 12, fig. 12) is definitely 

 quadrangular, the lateral longitudinal groove (pi. 14, 

 fig. 11) is narrower, and both ends are depressed. The 

 dorsal and ventral orifices of the vertical aqueducts for 

 the segmental blood vessels are not materially diflFerent 

 from those of the eleventh caudal. 



Thirteenth caudal.— This anteroposteriorly com- 

 pressed caudal (USNM 23794; pi. 14, fig. 12) differs 

 from the preceding caudal in being smaller, but other- 

 wise lacks distinguishing features. 



At least one small terminal vertebra was not repre- 

 sented in this caudal series. 



CHEVRONS 



Three chevron bones were found intermingled with 

 other skeletal elements of this mysticete, the largest of 

 which was articulated with the third caudal (table 15). 

 All three have strong haemal spines and large articular 

 facets on the transversely widened dorsal ends of the 

 lateral lamina. The haemal canal of all three chevrons 

 is smooth, without any indication of a median longi- 

 tudinal ventral ridge. 



Table 15. — Measuremenls (in mm.) of the chevrons, USNM SS794 



The third chevron (USNM 23794; pi. 1, fig. 2) has a 

 large elongated haemal spine, whose free ventral edge 

 curves convexly downward from the anterior end of the 

 haemal canal to its nearly straight vertical posterior 

 edge. The right articular facet is shorter and wider than 

 the left, which is more strongly attenuated as its anterior 

 end. 



The next largest chevron (USNM 23794; pi. 1, fig. 3) 

 is incomplete, the posterior half of the right lamina and 

 most of the left lamina are broken off. This chevron 

 has a short, broad haemal spine, truncated horizontally, 

 ventrally and a small obtuse anterobasal projection. The 

 vertical diameter of the haemal canal below the edge of 

 the articular facet is 22 mm. as contrasted to the 41 mm. 

 depth of the canal of the third chevron. The anterior 

 half of the right articular facet is similar in shape to 

 that of the third chevron. The measurements of the 

 haemal canal suggest that this may be the sixth chevron. 



The smallest (USNM 23794; pi. 1, fig. 4) of these 

 three chevrons has a somewhat shorter, ventrally thick- 

 ened haemal spine and a narrower anterobasal projec- 

 tion than the larger, more rounded posterobasal projec- 

 tion. The small haemal canal is higher (15 mm.) than 

 wide (10 mm.). Above the haemal canal the elongated 

 ovoidal articular facets are in contact medially. This 

 is tentatively identified as the seventh. 



