112 EDWARD PHELPS ALLIS jr., 



bular. The pseudobranch thus lies, at this stage of its development, 

 and also in younger stages, nearer the dorsal edge of the pterygo- 

 quadrate cartilage than the anterior edge of the hyomandibular. Its 

 hind end lies immediately in front of the anterior edge of the adductor 

 hyomandibularis muscle. 



Along the entire length of the ventral edge of the pseudobranch 

 there is, in 12 mm larvae (Figs. 5 and 11), a large and uninterrupted 

 arterial vessel, which receives several branches from the forming 

 tissues of the organ. Along the dorsal edge of the organ there is no 

 such continuous vessel, but there were, in certain of the transverse 

 sections, large venous or arterial spaces, whichever they may be, which 

 seemed to indicate that such a vessel was there in process of formation. 

 The ventral arterial vessel extends both forward and backward of the 

 region occupied by the pseudobranch. The anterior continuation of 

 it is the efferent pseudobranchial artery of Wright's descriptions, and 

 it was similarly designated in my own descriptions. The posterior con- 

 tinuation of it is certainly, at this age, the afferent artery of the organ. 

 It is not described by Wright, and was not noticed by me in the 

 more advanced larval stages examined in connection with my earlier 

 work, probably because my attention was wholly directed to the ar- 

 teries that entered the orbit. 



The efferent artery runs forward and mesially, at first along the 

 antero-lateral wall of the pseudobranchial cleft, and then, anterior to 

 the cleft, ventro-lateral to the basal part of the chondrocranium , in 

 the tissues immediately internal to the lining membrane of the dorsal 

 surface of the mouth cavity. The basal part of the chondrocranium 

 is here represented, in 12 mm larvae, by two wide longitudinal strips 

 of cartilage, one on each side of the head, the mesial edges of the two 

 strips inclining downward and mesially and being separated from each 

 other by the anterior end of the hypophysial fenestra, which, at this age, 

 is relatively much larger than in the adult. The ventro-mesial edge of 

 each of these strips of cartilage lies close against the lining mem- 

 brane of the brain. The dorsal edge inclines outward from the brain, 

 and leaves a certain space between it and the brain which represents 

 a part of the upper, lateral chamber of the eye-muscle canal of the 

 adult. Slightly in front of the anterior edge of the lateral wing of 

 the parasphenoid each of these strips of cartilage is cut out, on its 

 ventro-mesial edge, by a large notch which extends postero-laterally 

 into the cartilage and represents the united internal carotid and 

 efferent pseudobranchial foramina of the adult (1, fig. 10). The 



