IOWA ACADEMY OF SCIENCES. 73 



Anteriorly it is almost completely coalesced with the ossi- 

 fication of the quadrate, but posteriorly is is completely 

 free. In the advanced stage described by Hay the bone 

 has apparently fused with the ossified quadrate along 

 nearly the entire length of the latter. It may be ques- 

 tioned whether the slight ossification of the quadrate at 

 this stage is not derived from this membrane bone, Fur- 

 thermore, the ossification of the adult quadrate may be 

 derived, possibly, chiefly from the same source. No dis- 

 cussion of the homology of this bone will be attempted 

 here, as this subject will receive the attention of Professor 

 B. F. Kingsbury. 



At this stage of development the elongate otic capsules 

 are ossified but little except at their extreme anterior ends. 

 Kingsley has pointed out the fact that in the younger stage 

 described by him the anterior ends of the ear capsules 

 "project slightly beyond the point ol union with thecristae 

 trabeculai um." In the stage studied by me these projec- 

 tions are of considerable length and completely ossified. 

 Posteriorly they coalesce with the ossified portions of the 

 ear capsules, anteriorly they have every appearance of 

 being membrane bones. Bat one must have access to ear- 

 lier stages to definitely determine the origin of these pro- 

 jections. 



On the pterygoid bars are small membrane bones. 



THE VISCERAL ARCHES. 



On the lower jaw are two membrane bones, a dentary 

 and an angulare. I find no trace of an operculare, such as is 

 found in Necturus and some others. Ha3^has called atten- 

 tion to the peculiar relations of the membrane bone on the 

 ceratohyal bar in individuals of six inches in length, 

 essentially the adult condition. The thin splint which in 

 early stages lies on the median side of the ceratohyal later 

 comes to lie between two bars of cartilage, of which the 

 mesially lying one is interrupted at one point. Hay's 

 theory is that the mesial bar is formed by growth from the 

 the main bar around the ends of the bone. In my speci- 



6 I AS 



