l8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 



The extinct genus Cetotherinm and its allies represent the least 

 modified known stage (pi. 8, fig. 2). The anterior point of the 

 occipital shield lies decidedly behind the level of the orbit as well as 

 of the anterior margin of the articular portion of the squamosal, while 

 in front of the shield the parietals come into contact with each other 

 on the vertex, usually forming a distinct transverse band in front of 

 the occipital shield. Beyond the parietals the frontals also cross the 

 vertex as a wide band. They pass directly and broadly into the 

 supraorbital processes. A peculiarity which distinguishes the skull 

 of Cetotherhmt and its allies from that of all the other known baleen 

 whales is the gradual slope of the supraorbital process from the dorsal 

 level of the interorbital region downward and outward toward the 

 margin of the orbit. In all other members of the suborder the basal 

 part of the process is abruptly and conspicuously depressed below 

 the median dorsal level, so that the main outwardly-extending dorsal 

 surface of the process is more nearly horizontal in position. 



In the genus Rhachianectes a more advanced stage (pi. 8, fig. 3) 

 is represented. On comparing the diagrams it will be seen that this 

 stage is readily derived from the last. The principal peculiarities as 

 compared with Cetotherium are that the supraorbital process of the 

 frontal is broader at base (where it is abruptly depressed below the 

 dorsal level), the nasal is greatly enlarged, the nasal processes of the 

 maxillary and intermaxillary are lengthened, and the overthrust of 

 the occipital shield, while not enough to carry the front of the shield 

 beyond the level of the anterior margin of the articular portion of 

 the squamosals, has progressed forward sufficiently to push apart 

 the parietals on the vertex, where, however, these bones are trans- 

 versely united by a narrow band of interparietal. Interdigitation of 

 frontal, maxillary, intermaxillary and nasal is well marked, but it is 

 not sufficiently seconded by forward thrusting of the elements of the 

 braincase to involve the parietal. This bone is broadly exposed on 

 the surface of the braincase along the inner side of the temporal 

 fossa, its extremity just appearing on the vertex. It lies entirely 

 behind the nasals and the nasal branches of the intermaxillaries and 

 maxillaries. It is behind the frontal everywhere except in the region 

 where the supraorbital portion of the frontal joins the part which is 

 exposed on the vertex. Here the parietal sends forward a very short, 

 wide, triangular process which interlocks with the frontal. In many 

 important details of structure Rhachianectes stands apart from all the 

 known whalebone whales both recent and fossil. The nasals are 

 larger than in any other known cetacean. Notwithstanding the large 



