MUSEITM OF COMPARATIVE ZOOLOGY. 11 



The Basihyal (hhj/) is elongate, tongue-shaped, and posteriorly, at its 

 ■widest, is half as wide as long. Behind the more slender anterior por- 

 tion, glossohyal, there is a deep excavation or hole. Behind this, be- 

 tween the ends of the ceratohyals, in the proper basihyal portion, it 

 becomes much thicker, bulges downward, and has a couple of longitu- 

 dinal ridges near the middle. The narrower anterior portion is convex ; 

 posteriorly the border is concave, and with the sides forms two angles, 

 the hypohyals, which are produced to meet the first pair of cerato- 

 branchials. 



The basihyal of Hexanchus is broad and short, and the hypohyals are 

 not distinct. That of Heptabranchias is pointed in front and has small 

 hypohyals, according to Gegenbaur. Heptabranchias maculatus, how- 

 ever, 1 find to be nearly as in Hexanchus. The articulations in both 

 these cases diff"er from those described above, having more resemblance 

 to each other. 



Tlie Branchial Cartilages. 



Plate rS. 



Extrabranchials are not present. 



Basibranchials (h-hr). — The first or anterior of the series is distinct. 

 It is closely connected with the first, and loosely attached by ligament 

 to the second pair of hypobranchials. The second of the series is also 

 distinct, but closely joined with both second and third pairs of hypo- 

 branchials. The thii-d is fused with the corresponding pair of hypos, 

 has an oblique and indistinct longitudinal division, and is closely joined 

 with the fourth, which in turn is fused with the fifth. The pairs of 

 hypobranchials corresponding to the last two are mere processes on their 

 sides. As in other Selachia, the last of the series ends in a long spine- 

 like process. In transverse section the anterior four are triangular, flat 

 above, keeled beneath. 



A complete series of basibranchials in unsegmented condition would 

 indicate lower rank than such as that figured by Gegenbaur (PI. XVIII. 

 fig. 1) in Heptabranchias. Excepting the last of the series, higher rank 

 of a genus is apparently accompanied by reduction or loss of basibran- 

 chials (see Spinax, Scijllinm, Galens, and others). 



Professor Gegenbaur's figure of Heptabranchias is not to be taken as 

 characterizing the genus, since in any of the genera there are hardly two 

 species alike in respect to these cartilages. In most cases the estimate 

 of the value of these and other portions of the skeleton, in connection 



