426 C. H. DANFORTH 



divisions, from which the ultimate filamentar arteries (about 

 twenty) arise. 



This vessel and the afferent hyoidean artery referred to above, 

 apparently correspond to the similar arteries in Amia and the 

 young trout. These vessels Allis ('00) and Maurer ('88) believe 

 on embryological grounds to be the true afferent arteries of the 

 first and second arches, and this would seem to be the impli- 

 cation of Greil ('03) in reference to elasmobranchs. But Wright 

 ('85) working with Lepidosteus, and Silvester ('04) with teleosts 

 are inclined to other interpretations. It can hardly be profit- 

 able to discuss this question in connection with Polyodon until 

 something of its embryology is known. 



The hypobranchial arteries posterior to the hyoidean are more 

 variable and tend to become asymmetrical. The recurrent 

 vessel (fig. 4, a.hb.2) from the second gill, when fully developed, 

 on entering the hypobranchial region passes behind and dorsal 

 to a small accessory tendon from the m. sternohyoideus to the 

 second branchial cartilage. Here it gives off an anterior branch 

 {a. Ihb.) which passes dorsal to all other arteries and tendons 

 to supply the lateral lobe of the thyreoid and the articular sur- 

 faces of the anterior basibranchials. At about the same place 

 it also gives rise to a posterior branch {a.lW .) which runs back 

 dorsal to the second afferent artery to reach the ventral aspect 

 of the third basibranchial cartilage. The recurrent vessel itself, 

 turning slightly backward and inward, presently gives off a third 

 branch (a.sthy.). This is to the sternohyoideus (m.sthy.), the 

 tendon of which it follows along the dorso-medial side. It 

 generally passes laterad of the m. branchiomandibularis (?w. 

 hmd.), but may go mesad of it. In one specimen, unfortunately 

 injured, the vessel seemed to encircle the muscle. The vessels 

 of the two sides meet behind on the belly of the sternohyoideus 

 but remain more or less distinct. After giving off these three 

 branches the main trunk of the recurrent artery reaches the 

 median line ventral to the aorta, where it meets a corresponding 

 artery (if present) from the other side. Frequently the two 

 vessels {a.rnhb.), usually of unequal size, run forward side by 

 side without uniting. They supply the median lobe, and possi- 



