126 ALUS. [Vol. XVIII. 



along the ventral surface of the triangular portion, a sort of pocket 

 being often formed between the two portions. Opposite this 

 pocket the mesial edge of the groove is cut away, as on the first 

 arch, and the aft'erent artery of the arch here reaches the ventral 

 surface of the hypobranchial. Anterior to the pocket, the mesial 

 edge of the ventral surface of the element often projects poste- 

 riorly in a nearly longitudinal position, and forms a more or less 

 developed process which lies immediately ventral to the artery of 

 the arch. In the specimen used for illustration this process was 

 but slightly developed, and, by error, the line marking its hind end 

 has been omitted in the figure. In all the figures the hypobranchials 

 are pulled somewhat apart so as better to show the basal line. In 

 its natural position this part of the second hypobranchial closely 

 adjoins, in the mid-ventral line, the corresponding edge of its 

 fellow of the opposite side of the head, a slit-like space only being 

 left between the two edges. The opposing surfaces of the two ele- 

 ments here partly enclose the truncus arteriosus. 



The proximal end of the element is capped with cartilage and 

 articulates with the ceratobranchial of its arch. 



The Third Hypobranchial (HB, III) may be said to consist 

 cf a body, and one or two thin laminar processes. The body of 

 the piece has a wide and thick posterior end, from which it tapers 

 rapidly forward, in curved lines, into a long and slender anterior 

 end. The lateral edge of its posterior third is thin ; the cor- 

 responding portion of its mesial edge being thickened. The dorso- 

 mesial corner of the hind end of the element projects upward and 

 mesially, as a relatively stout eminence, which nearly meets in the 

 middle line, dorsal to the line of basal elements, the corresponding 

 part of the hypobranchial of the opposite side of the head. The 

 external surface of the element is convex, and is presented dorso- 

 laterally. The internal surface is concave and is presented ventro- 

 mesially. The hind edge of the bony part of the element is 

 rounded, and is capped with a thick cap of cartilage, the free, 

 posterior edge of which presents three regions separated by more or 

 less pronounced angles. The lateral and mesial ones of these three 

 regions are larger than the middle one, and articulate, respectively, 

 with the ceratobranchial of the third arch, and with the antero- 

 lateral edge of the small median piece of cartilage that seems to 

 represent the fused hypobranchials of the fourth pair of arches. 



