80 ALLEN 



I. INTRODUCTION. 



Material. — The Ganoids used for this problem consisted of 

 Polyodon spathula (spoon-bill cat), Lcpisostciis tristocchus (alli- 

 gator gar), JLeptsosteus ossctis (spike-bill gar), and Lepisostetts 

 platostomus (duck-bill gar), which are the most common Ganoids 

 found on the mud bars and in the sloughs of the Ohio and ATis- 

 sissippi rivers about Cairo, Illinois. The fish were mostly taken 

 through seining by moonlight ; for it is at night that they leave 

 the channels to do their feeding on the bars. L. trist(xchus is 

 by far the largest of the LepisosfeidcB and I at first hoped to use 

 it as a type for the Rhomboganoidea, but found it necessary 

 to give it up for the following reasons : L.. tristcechus could 

 only be obtained during the fall of the year. Other things being 

 equal it was always able to force its way through the seine, and 

 the large specimens were found too massive to inject and pickle 

 conveniently. Consequently most of the injections and dissec- 

 tions were made on Z. osseus, which differed from Z,. -platos- 

 tonius in no important detail, but which was found to be more 

 suitable for these investigations. 



Most of the dissections were made during what spare time I 

 had, while engaged in somewhat similar work for Mr. E. P. 

 Allis, Jr., at Cairo, Illinois, and the work was continued when 

 called by him to the Stanford Marine Laboratory at Pacific 

 Grove, California. 



Method of Procedure. — As in my previous studies Hoyer's 

 lead chromate gelatin mass and a Berlin blue gelatin mass 

 were the injecting media used. Ordinarily the blood vessels 

 were first injected with the blue mass, and the subcutaneous 

 vessels were afterward filled with the yellow. Instead of making 

 my own soluable blue as formerly, I found that what is sold by 

 druggists as Tiernan's soluble Prussian blue was equally satis- 

 factory and involved far less labor. Since all that is necessary 

 is to add 200 c.c. of concentrated Prussian blue solution, pref- 

 erably hot, to a hot solution of 25 gr. of gelatin that had been 

 expanded in 100 c.c. of water. 



In nearly every case the head was severed from the body a 

 short distance behind the shoulder girdle, the intestine cut at 

 the rectum, and the viscera pulled out with the head. With 



