. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. 402 BuUetin Museum of Comparative Zoology, Vol. 142, No. 4. Figure 6. Cerofosfefhus bicornh. Lateral view of jaws in varying stages of protrusion (for explanation see text). until after more than half of the swinging movement is completed and the premaxil- laries are considerably protracted. Up to this point movement of the premaxillaries is equal to that of the rostral; from this point on, however, the premaxillaries move rel- atively further forward than do the rostrals. Thus not only does the rostral slide forward
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. 402 BuUetin Museum of Comparative Zoology, Vol. 142, No. 4. Figure 6. Cerofosfefhus bicornh. Lateral view of jaws in varying stages of protrusion (for explanation see text). until after more than half of the swinging movement is completed and the premaxil- laries are considerably protracted. Up to this point movement of the premaxillaries is equal to that of the rostral; from this point on, however, the premaxillaries move rel- atively further forward than do the rostrals. Thus not only does the rostral slide forward relative to the cranium, but the premaxil- lary processes slide forward relative to the rostral; Alexander (1967b) found a similar movement present in Atherina but absent in the cyprinodontids Aplocheilus and Fundulus. Alizarin preparations of various cyprinodontids examined by me failed to exhibit this movement. The screw move- ment evidently also causes dilation of the lower limbs of the premaxillary, causing the lateral series of enlarged premaxillary teeth (which are directed sideways when the mouth is in resting position) to point straight ahead. As stated by Alexander (1967b: 241) protrusion of the jaws must be accompanied by screw movement of the maxillary, sc that the internal hooks and consequenth rostral bone are moved anteriorly. In Ceratostethiis, however, at least some pro- trusion occurs before the screw movemeni begins. Furthermore, it is possible to causf considerably more protrusion (about 21 percent more) after the rostral has ceasec to move forward by simply continuing t( depress the lower jaw. If this is done, the premaxillary tilts upwards posteriorly sc that the mouth opening is slightly down- wards instead of vertical. Wliile it i; doubtful that the jaws are normally pro truded so far, this observation indicate; that depression of the jaws can act un accompanied by screw movements of tht maxillary to bring the premaxillaries for ward. In cyprinodontoids t
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