NO. 5 TELESCOPING OF THE CETACEAN SKULL 23 



maxillaries seems to have been either more pronounced than the 

 forward thrust of the occipital or to have reached advanced stages 

 of development earlier in the process of telescoping than the period 

 when the forward advance of the occipital became conspicuous ; in 

 the othen (pi. 6, figs, i, 2; pi. 7, fig. 3) the discrepancy between the 

 two movements is less obvious, and their share in the remodeling of 

 the skull appears to have been less unequal. One of the most notice- 

 able results of the first tendency has been to carry the maxillary 

 back in such a manner that the frontal, except in the orbital rim, is 

 practically excluded from view in the region lying above and behind 

 the eye ; at most it may be visible as a band of varying width extend- 

 ing upward and backward along the margin of the temporal fossa 

 behind but not over the orbit (pi. 5, figs. 4, 5; pi. 7, fig. 4). The 

 second tendency, on the contrary, brings about conditions in which a 

 relatively wide area of the frontal is visible in the region directly 

 above the orbit (pi. 6, figs, i, 2 ; pi. 7, fig. 3). Each of these tendencies 

 has been worked out in great detail ; the dominant maxillary thrust is 

 particularly well shown in Stenodelphis (pi. 7, fig. 2), the more 

 balanced condition in Kogia (pi. 7, fig. 3). Contrary to what might 

 be expected it is the second tendency which has led to the develop- 

 ment of the cetacean skulls (pi. 6, fig. 2, and pi. 7, fig. 3) whose 

 structure is most fundamentally removed from a characteristic ter- 

 restrial mammalian type (pi. 7, fig. i) . 



The least modified structure positively known to exist occurs in 

 the genera Agorophin<, (pi. 5, fig. 2), ArchcEodclphis (pi. 5, fig. 3), 

 and Xenorophus (pi. 5, fig. 6), the first and third from the Eocene or 

 Oligocene of South Carolina, the second perhaps from the same 

 region and horizon.^ The skull is broad and short (for dorsal view 

 of Agorophius see True, Remarks on the type of the fossil cetacean 

 Agoropluus pygmcvus (Miiller), Smithsonian Inst., Publ. No. 1694, 

 1907. For other figures of ArcJuvo del phis see G. M. Allen, Bull. 

 Mus. Comp. Zool., Vol. 65, No. i, August, 1921). The process of 

 telescoping is well advanced and strictly of the modern odontocete 

 type. The occipital, however, contrary to the conditions existing 

 in all other known toothed cetaceans except the zeuglodonts, has not 

 yet extended far enough forward to meet the frontal. The parietals 

 are therefore still present on the vertex of the braincase, where they 

 form the roof of a short but obvious postorbital constriction the width 



* An e:-sentially similar structure may be present in Patriocctus from the 

 Oligocene of Austria ; but the published facts concerning this genus are not 

 sufficiently conclusive to warrant any generalizations (see pp. 42-44). 



