THE QUARTERLY REVIEW OF BIOLOGY 



backward interdigitation of the median 

 rostral elements. Nevertheless, skulls of 

 Miocene cetotheres from later geological 

 stages do not lend any support to this 

 assumption, for Cetothermm and related 

 genera exhibit a less advanced stage in 

 the forward overthrust of posterior occip- 

 ital elements and a pronounced inter- 



5. be 



Exfoc, 



Fig. 17. Dorsal View of Skull of Cetotherium 



rathkii, Upper Miocene, Russia 



(After Brandt) 



digitation of rostral and cranial elements. 

 It is obvious that our available material 

 is too meagre for more than vague general- 

 ization on the lines of descent and that 

 any discussion of the history of the 

 mysticetes must be confined to types 

 arbitrarily selected to illustrate certain 

 features of this general evolutionary 



process. There is evidence that several 

 phyla of cetotheres advanced along similar 

 lines. In one direction there is a per- 

 sistence of the dominant forward over- 

 thrust of posterior occipital elements, 

 and an absence of rostral and cranial 

 interdigitation. This is the type of 

 remodeling that led up to the modernized 

 balaenids. In the other direction, the 

 forward overthrust of the posterior occip- 

 ital elements was coupled with a backward 

 interdigitation of the median rostral 

 elements, as a result of which the postero- 

 internal extremities of the maxillaries, 

 the premaxillaries, and the nasal bones 

 became suturally united with the frontals 

 on the vertex of the interorbital region. 

 Some of the extinct cetotheres and all of f 

 the living balaenopterine whales exhibit 1 

 this type of telescoping. Various inter- 

 mediate stages of these two general [ 

 trends occur among the cetotheres. 



BALAENIDAE 



Winge (1918) was the first to suggest 

 that the balaenids represent the most 

 primitive known mysticetes, although 

 at the time this was written no right 

 whales older than Pliocene had been 

 described. That the Balaenidae were 

 already differentiated as a family before 

 the Lower Miocene is made certain by the 

 occurrence of an already specialized form, 

 Morenocetvs parvus (Cabrera, 1916), in the 

 Patagonian marine formation of Chubut, 

 Argentine Republic. The dominant for- 

 ward overthrust of the posterior cranial 

 elements has carried the apex of the 

 supraoccipital shield to the anterior 

 interorbital region, excluding the parietals 

 from the vertex of the skull, and reducing 

 the median exposure of the frontals to 

 a narrow strip behind the nasal bones. 

 The sutures show that the nasal bones, 

 the premaxillaries, and the maxillaries 

 occupied the usual balaenid position 



