28 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 7O 



a strong tendency to unusually close contact of the two bones, would 

 be expected tO' occur ; but no such conditions are found, except, per- 

 haps, among the ziphiids (pi. 5, fig. 5; pi. 7, fig. 4), where the for- 

 ward curling of the posterior extremity of the maxillary suggests 

 the possibility of development through secondary occipital encroach- 

 ment on a structure previously resembling that of StenodelphM (pi. 7, 

 fig. 2). With this possible exception the developmental tendency of 

 all these recent genera appears to have been less strongly dominated 

 by the backward thrust of the maxillary than in Inia, Lipotcs, Steno- 

 delphis, Monodon, and Delphinapfcriis; the braincase has enlarged 

 more freely, and the area for attachment of the neck muscles has been 

 increased by some forward extension of the occipital beyond the 

 level of the condyle (compare the occipital of Dclphimis, pi. 5, fig. 4, 

 with that of Stenodclphis, pi. 7, fig. 2). This combination has given 

 rise to the most efficient and successful of all cetaceans, the recent 

 dolphins, a type which shows great plasticity and at the same time 

 little tendency toward extravagance and gigantism. Some of the 

 more conspicuous results to which this plasticity has led are as 

 follows. In the ziphiids (the fundamental similarity of whose skull 

 to that of the delphinids may be seen on comparison of pi. 5, figs. 4 

 and 5) the beak tends to be deepened and solidified into a peculiar 

 and characteristic form; the upper teeth have disappeared as func- 

 tional organs ; most of the lower teeth have similarly disappeared 

 while the few that remain have been enlarged and specialized ; the 

 region of contact between the occipital and frontal is unusually ele- 

 vated and is situated not far behind the orbital level (see the point 

 marked + in fig. 5, pi. 5, and fig. 4, pi. 7) ; the entire posterior part 

 of the skull seems to have advanced forward ; the maxillary appears 

 to turn secondarily forward in its hindermost and uppermost part 

 (see especially pi. 5, fig. 5) ; the lacrimal is free from the jugal ; the 

 pterygoid is greatly enlarged, but its reduplications are represented by 

 mere ridges ; the hindermost ribs are supported by transverse pro- 

 cesses which have peculiarities making them seem to be perhaps not 

 serially homologous with those which support the others, the change 

 taking place abruptly between two contiguous vertebrae ranging in 

 position from the 6th and 7th (Hyperoodon; two processes visible on 

 the 7th dorsal vertebra) to the loth and nth (Bcrardiits). In the 

 delphinoids the beak may be lengthened or broadened, but it is never 

 deepened and solidified as in the ziphiids ; the lacrimal is fused with the 

 jugal ; the pterygoid is not specially enlarged and its internal reduplica- 

 tion is well developed (pi. 5, fig. 4) ; the hindermost ribs are sup- 



