THE SKELETON OF TRIMERORHACHIS 297 



the ilia a primitive or an acquired character ? In such animals as 

 the mosasaurs and ichthyosaurs, when the connection of the pelvis 

 with the sacrum was lost, the ribs disappeared; they did not revert 

 to their primitive forms. On the other hand, Necturus has its 

 sacral ribs differing from the preceding ones only in their slightly 

 larger size, and Necturus certainly has not had an exclusively 

 aquatic ancestral line from the fishes. Necturus also has its lower 

 pelvic bones, as well as the mesopodials, cartilaginous, and their 

 ancestors at some time must have had them ossified. 



Not only was Trimerorhachis a purely aquatic animal, but, in 

 much probability, it was also, like Necturus, perennibranchiate. 

 Lying just within the angle of the left mandible there is the end of 

 a flat bone, 11 mm. in width, with a more slender anterior end 

 directed forward and inward, that can only be an unusually large 

 hyoid or epibranchial bone. Back of it there is the end of what 

 appears to be a smaller one. On either side of the first vertebrae, 

 directed forward and outward, there are the ends of three small, 

 rib-like bones; I do not know what they are. The length of the 

 skull in this specimen (No. 1,271) is 165 mm.; its width posteriorly 

 the same. The length to the beginning of the tail, allowing for the 

 sinuosities, is 550 mm. The length of the entire animal — and the 

 specimen is one of the largest — must have been less than one meter. 



