THE QUARTERLY REVIEW OF BIOLOGY 



and the Tortonian Siphonocetus prisms, 

 and Earietobalaena palmeri (Kellogg, 192.4), 

 on which the nasal bones, the premaxil- 

 laries, and the rudimentary postero- 

 internal extremities of the maxillaries, 

 while suturally united with the mesial 

 projections of the frontals, are situated 

 for the most part anterior to the level of 

 the preorbital angles of the supraorbital 

 processes. The apex of the supraoccipital 

 shield extends forward beyond the extrem- 

 ities of the zygomatic processes and the 

 posterior margins of the supraorbital 

 processes. The frontals are broadly 

 exposed in the interorbital region, and the 

 parietals are shoved forward across the 

 frontals, meeting on the mid-line to form 

 a short sagittal crest. 



The interdigitation of the rostral and 

 cranial portions of the skull, in so far as 

 substantiated by described specimens, is 

 first observed in the Lower Miocene 

 "Cetotberium" furlongi (Kellogg, 192.5), 

 which was found in Monterey County, 

 California. The backward interdigitation 

 of the mesial portion of the rostrum carried 

 the ascending processes of the maxillaries 

 and the premaxillaries, as well as the 

 nasal bones, beyond the level of the 

 preorbital angles of the supraorbital 

 processes. In consequence of this back- 

 ward movement, the facial portion of the 

 skull becomes "dished in" mesiallyin some 

 of the succeeding cetotheres, the anterior 

 borders of the supraorbital processes 

 curving forward and outward. The Tor- 

 tonian Cephalotropis coronatus (Case, 1904), 

 the Anversian Mesocetus longirostris (Van 

 Beneden, 1886), and the Sarmatian Ceto- 

 tberium rathkii (Brandt, 1873), anc ^ Ceto- 

 therium helmersenii may be cited as examples 

 of this stage in the telescoping process. 

 Some of these species were no larger than 

 the new-born young of living finbacks. 



Skulls of species referable to the Miocene 

 genus Cetotherwm were characterized in 



part by forward curving supraorbital 

 processes, by the facial position of the 

 nasal bones, by anteriorly directed zygo- 

 matic processes with peculiar elongated 

 glenoid articular surfaces, as well as by 

 long slender mandibles. A peculiarity 

 that distinguished Cetotherium and related 

 genera from other Miocene cetotheres 

 is the presence of a sharply defined anterior 

 temporal crest, which seems to foreshadow 

 the abrupt depression of the basal portion 

 of the supraorbital process seen in balaen- 

 opterine whales. Cetotheres belonging 

 to this small group likewise possess the 

 least modified braincase, for the parietals 

 come in contact with each other in the 

 intertemporal region, and the apex of 

 the supraoccipital shield lies behind the 

 level of the supraorbital processes. 



The interdigitation of the rostral and 

 cranial elements is well marked in a 

 mysticete from Monte Titano, San Marino, 

 Italy, which Capellini (1901) named 

 "Aulocetus" sammarinensis. The forma- 

 tion from which this specimen was 

 obtained has been allocated by some 

 geologists to the Langhian stage and by 

 others to the Helvetian. The narrow 

 nasal bones have not been shortened, 

 although they lie almost entirely behind 

 the level of the preorbital angles of the 

 supraorbital processes of the frontals, and 

 are as long as or longer than the breadth 

 of the extremities of the latter. A short 

 intertemporal region with broad sagittal 

 crest is retained, and the apex of the 

 rounded supraoccipital shield is not ex- 

 tended forward to the level of the extremi- 

 ties of the outward bowed zygomatic 

 processes. 



In the preceding brief resume of the 

 cetotheres the writer has endeavored to 

 select types that show the main structural 

 features of their evolutionary history, 

 and many equally important species have 

 been omitted. Capellini, Strobel, Portis, 



