422 C. H. DANFORTH 



The main stem (a.fiLa.) ascends to the top of the filament, giv- 

 ing off throughout its whole length a series of short cross pieces 

 which reunite in an irregular network (a.fil.x.) from which the 

 'afferent transverse filamentar arteries' {a.fil.tr.) take origin. 

 Each of these supplies several of the flap-like folds of respiratory 

 epithelium {rsp.) on either side of the filamentar cartilage {c.fll.). 

 From the ultimate capillaries the blood is returned directly 

 to the efferent filamentar artery (a.fil.e.) which descends on the 

 other side of the cartilage to pass through a notch in its base. 



It is stated above that the efferent branchial artery of the 

 fourth gill varies from the corresponding vessels anterior to it. 

 This divergence is correlated with other modifications in the 

 region. The fourth gill, unlike the others, is not a complete 

 holobranch. Its anterior hemibranch (fig. 9, hb.a.) is entire but 

 the posterior, {hb.p.} although constantly present, extends only 

 slightly into the epibranchial region where a fusion has taken 

 place, obliterating the dorsal half of the cleft. The ventral 

 half of the cleft, however, remains open. This simple but un- 

 usual condition has proved misleading to several writers. Van 

 Wijhe (op. cit.) says that each of the four first gill-arches bears 

 on its outer side a whole gill. Jordan ('99) diagnosing the family 

 Polyodontidae, says 'gills 4|'. Finally Allen twice states (I.e., 

 p. 106, p. 108) that/' the fourth or last branchial arch has but 

 one row of filaments, and is therefore a hemibranch." Strictly 

 speaking all of these descriptions are incorrect. Apparently 

 Van Wijhe failed to notice the absence of the dorsal half of the 

 fourth hemibranch, while Allen overlooked the presence of the 

 ventral half. Jordan's statement is probably based on the car- 

 tilaginous arches and not on the gills themselves. A consider- 

 able number of specimens examined by the present writer proved 

 very uniform in this particular except that in a young one, 74 

 mm. long, the filaments were found to be rudimentary in the 

 epibranchial region of all the gills. In its lower half the fourth 

 gill is like those in front of it and its efferent artery gives off the 

 characteristic hypobranchial vessel in the usual manner. 



Near the point where it rounds the articulation between the 

 epi- and cerato-branchial cartilages, the fourth efferent artery 



