NO. 1211. NOTES ON THE ZEUGLODON— LUCAS. 831 



that the caudals increase rapidly in length fioni the ninth forward, a.s 

 is shown by the following measuronjonts: 



Antero-posterior length of centrum of lifth vertebra, 2% inches; 

 sixth vertebra, 3i inches; seventh vertebra, 6 inches; eighth vertebra, 

 S inches; ninth vertebra, 10 inches; twelfth vertebra, 18 inches. 



The length of the loft os innominatum, allowing 5 nnn. for the broken 

 portion, is 245 mm. from the anterior end of the pectineal process to 

 the posterior end of the ischium. The length of the femur is 190 mm. 



The material in the collections of the United States National Mu- 

 seum enables us to add a little to the diagnosis of the Basilosaundce^ 

 and to differentiate the genera BasUoscmri/s- and Dorudon. The family 

 may be thus characterized: Dentition, i. \ c, } pm. | m. §?; incisors 

 caniniform, lower molariform teeth deepl}^ serrate on one or both 

 edges; premaxillaries and maxillaries elongate; cervicals with com- 

 pressed centra, not ankylosed, but so interlocked by processes as to be 

 practically immovable; anterior ribs more or less expanded distally; 

 scapula with a slender coracoidal and elongate acromial process, both 

 directed forward as in Cetacea; forearm movable on humerus; meta- 

 carpals and phalanges elongate as in Otariidae; pelvis and hind limb 

 vestigial; femur with a third trochanter. 



BaHilosaurua. — Molariform teeth serrate on both edges, save last 

 lower molar and first upper pre-molar; bodies of lumbo-caudals much 

 elongated, with low neural arch over center of centrum. 



Dorudon. — Molariform teeth serrate on posterior edge only; bodies 

 of lumbo-caudals short with high neural arch over anterior part. 



In conclusion it ma}' be said that the writer believes Ba.nlosaurus 

 left no successors, but considers that like Ilesperornis among birds 

 this highly modified form represents a side branch of the ceto-phocine 

 tree. 



EXPLANATION OF PLATES, 

 Plate V. 

 External aspect of right os innominatum of Bas'dosaurus ectoldcs, reduced. 



Plate VI. 

 Internal aspect of left os innominatum of Basilosauruii ccloidcs, reduced. 



Plate VI I. 

 Posterior and anterior aspects of right femur of Basllomurus cetoiden, reduced. 



