66 



THE ELASMOBRANCH FISHES 



found. The rudimentary arch in the embryo of Heterodontus consists of at 

 least two segments, which in the adult may still be seen, welded more or less 

 closely to the fifth branchial arch (fig. 72). Rudiments of such structures are 

 of interest in forms like Heptanchus and CJilamydoselachus in which an un- 

 usual number of arches becomes functional in the adult. That still other rudi- 

 ments are present in the embryo indicates that ancestral forms possessed a 

 number of arches exceeding that of present-day types. 



Fig. 73. Median ventral basibranchial cartilages. A. Chlamydoselachus. (From Goodey.) 

 B. Heterodontus francisci. 



1)1)., basibranchial; hh., basihyoid ; ch., ceratobranchial ; g.p., glossal process; hh., liypo- 

 branchial; mi)., median piece. 



Figure 50b represents the area of rudimentary arches in Heptanchus macu- 

 latus. It will be observed that a slight asymmetry is shown which gives a some- 

 what greater development of the midventral region on the right than on the 

 left side. Through this asymmetry the rudimentary arch and adjoining area 

 on the right side are more highly developed than on the left. 



Upon examination of the ventral side of the rudimentary arch on the right 

 side I found certain round and pointed rays (/•., fig. 50b) arising from the 



