NO. 5 TELESCOPING OF THE CETACEAN SKULL 27 



on side of braincase, but area of temporal fossa considerably reduced 

 through the drawing inward of the large' articular processes of the 

 squamosal. In Monodon and Delphinapterus the region of contact 

 between the occipital and frontal is about as far behind the level of 

 the orbit as in Sqiialodon or Stenodelphis, but the braincase has 

 become so much enlarged that it causes a marked reduction of the 

 temporal foSsa (the fossa, when skull is viewed from behind, is 

 obviously smaller than the combined area of the occipital condyles 

 and the foramen magnum) and very considerably reduces the area 

 of the frontal exposed on its side. A condition whose main features 

 apparently resemble those found in Monodon and Delphinapterus 

 seems to occur in most of the Miocene dolphins. Such material 

 as I have examined (representing several genera) from the Calvert 

 formation of Maryland agrees with these two living genera and 

 with the published figures of skulls from the Antwerp Crag in the 

 great backward extension of the maxillary behind the orbit and the 

 posterior position of the region of contact between the occipital and 

 frontal. In these extinct dolphins the size of the temporal fossa 

 appears to be reduced as compared with the condition seen in 

 Sqnalodon, Inia and Lipotes; but the state of preservation of the 

 specimens makes it unsafe to generalize on the subject. The area 

 of the frontal exposed on the side of the braincase also seems to have 

 undergone the corresponding restriction. It must always be remem- 

 bered, however, that characters of this kind are easily obscured by 

 the crushing and other injuries which fossils usually have suffered. 

 Returning to the recent genera we find that in all of those not pre- 

 viously mentioned among the types showing dominance of the maxil- 

 lary thrust (that is in all except Inia, Lipotes, Stenodelphis, Monodon, 

 and Delphinapterus) the posterior border of the maxillary has not 

 been carried very far behind the level of the orbit, and at the same 

 time the region of contact between the occipital and frontal has been 

 advanced to a position not conspicuously behind the posterior orbital 

 level (compare Delphiniis, pi. 5, fig. 4 with Stenodelphis, pi. 7, fig. 2). 

 This condition might be interpreted as a regression of the hinder 

 maxillary edge under the influence of forward pressure of the oc- 

 cipital ; but I am inclined to believe that in most instances it really 

 indicates a course of development independent from that which was 

 followed by those members of the group in which the maxillary 

 reached its extreme backward extension. Had the hinder margin 

 of the maxillary been secondarily pushed toward its original posi- 

 tion by a forward-advancing occipital some overlapping, or at least 



