IOWA ACADEMY OP SCIENCES. 69 



THE MEMBRANE BONES IN THE SKULL OF A 

 YOUNG AMPHIUMA. 



BY H. W. NORRIS. 



The specimen upon which the following description is 

 based was 51 mm. in length. In the state of development 

 of the skull it appears to be intermediate between the two 

 stages described and illustrated by Winslow with figures 

 of wax models. The chondrocranium appears to be almost 

 exactly like that of the older specimen described by him, 

 except that the ossification in my specimen is less 

 advanced. The specimen was sectioned and a reconstruc- 

 tion of the skull was made after Born's method. 



THE CRANIUM. 



Ethmoidal Begion. Of the bones found in the adult in 

 this division of the skull only the nasals are wanting at 

 this stage of development. The premaxilla has essentially 

 the adult form described by Wiedersheim and by Hay. 

 Along the border of the alveolar process of the premax- 

 illa are about thirty teeth, of which ten are attached to 

 the bone, five on one side and four on the other, with an 

 unpaired tooth in the middle. Mention should be made 

 here of the small unpaired cartilage situated in the roof of 

 the mouth ventral to the palatine process of the premax- 

 illa. Hay's theory is that "the anterior lobes of the tra 

 becular cornua * * * grow downward until they meet 

 below the palatine process and then coalesce." Afterward 

 the part below the plane of the vomers becomes cut off and 

 forms the unpaired piece. In my specimen this unpaired 

 piece, wdiile very distinct, has not yet developed a hyaline 

 structure. It is in a semi-membranous condition, but 



