[9] THE OSTEOLOGY OF AMIA CALVA. 755 



external view. Similar bone-plates, provided with fine little teeth, such 

 as those just described for the parasphenoid, are found upon the pala- 

 tine, upon the three pterygoids, and upon the splenial of the mandible. 

 After the excellent investigations of Leydig'^ and O. Hertwig^^ a par- 

 ticular reason is hardly required if I place the parasphenoid and the 

 vomer, as ossifications of the mucous membrane of the mouth, opposite 

 the dermal ossifications and the true ossifications of the skuU. 



In respect to this, it seems to me that the condition found to exist 

 in Folypterus is of peculiar significance ; \ in this form, according to 

 Leydig's investigations, all the bones of the buccal cavity are covered 

 over by the epithelial layer solely. The Amia, where the ossifications 

 beneath the epithelium are likewise covered by a layer of connective 

 tissue, constitutes an excellent example, so far as this condition is con- 

 cerned, of the transition stage between this form and the majority of 

 bony fishes, in which the parasphenoid and vomer are hidden beneath 

 the thick mucous membrane of the mouth. 



After the cranium has been skeletonized, the parasphenoid and the 

 parial vomer can be easily discerned. 



The parasphenoid (Plate I, Fig. 2, ps.) is aflat bone, having the form of 

 a cross. Its stem extends from the hindermost extremity of the skull 

 to the antorbital, and very near its middle it gives off two branches, 

 which extend laterally and upward alongside the postorbital, and form 

 the posterior boundary of the orbit. 



The posterior extremity of the parasphenoid is deeply cleft, thus 

 allowing a small triangular portion of the basi cranii, represented by 

 the basioccipital, to come into view upon a basal aspect of the skull. 

 That part of the bone which is provided with teeth, and which in 

 different individuals varies with regard to its anterior and posterior 

 extension, lies mesially between the two branches. 



In front of the parasphenoid are found the two vomers (Plate I, Fig. 

 2, vo.), articulating with each other iu the middle line. They are flat 

 osseous plates, placed longitudinally, with their anterior thirds armed 

 with stout teeth. Their posterior moiety covers the anterior part of the 

 inferior aspect of the parasphenoid. 



If the statement that the parasphenoid originally bore teeth over its 

 entire surface be correct — and so many facts have been adduced in its 

 favor that its correctness can hardly be doubted — the overlapping of 

 the vomer on this bone must be a primitive state of affairs. In fact, if 

 one compares this condition of Aniia, with its parial vomer, with the ar- 

 rangement in bony fishes, where the vomer is known to be always non- 

 parial, hardly a doubt but that Amia represents the primitive condition 

 remains. 



^*W. Leydig, Beitrag z. mikroskop. Anatomie v. Polypterus. Zeitschr. fur wiss Zool., 

 Bd. V. 

 '"O. Hertwig, Das Zahnsysiem der AmjpMbien. f. mikroskop. Anatomie. Bd. XI, suppl. 



