18/0.] PROF. T. H. HUXLEY ON CERATODUS FORSTERI. 



37 



ventral line, between the ends of the ventral pieces, lie two cartilages — 

 an anterior elongated and spindle-shaped {M.B^), which is connected 

 by a strong ligament with the median piece of the hyoid arch, and a 

 posterior rounded nodule (M.B-j at some distance from the foregoing. 

 The fifth arch consists of a single piece of cartilage curved forwards 

 and united with the principal branchial of the fourth arch, both 

 above and below; in front of it, close to the ventral end of the fifth 

 arch, was a small nodule of cartilage, which is probably a rudimen- 

 tary sixth arch (6). 



Fi-. 7. 



a > 



Ceratodus forsteri. Dorsal aspect of the skull. 

 a, the anterior end of the chondrocraniuin ; b, the antorbital process of the 

 chondrocranium ; c, its suprabranchial expansion ; d, lateral elevations of 

 the occiput ; and e, median ridge, with the surface for the attachment of the 

 anterior fin-ray ; /, articular surface for the second fin-ray ; A, anterior 

 median bone ; B, posterior median bone ; C, inner lateral bone ; C, distinct 

 ossification on the jiosterior extremity of this bone ; B, outer lateral bone 

 (squamosal) ; E, a separate ossification on the left side ; 0;j, operculum ; 

 Or, orbit ; Sb.o, suborbital bones ; 2, the posterior ujiper labial cartilage. 



As Dr, Giinther has pointed out, the osseous shield which covers 

 the dorsal aspect of the skull consists of two median bon«s (fig. 7, 

 A, B), one anterior and one posterior, which he terms "ethmoid" 

 and "scleroparietal," and of two pairs of lateral bones, the "frontals" 

 internally (fig. 7, C) and the "tympanic laminse " externally (fig. 7, 

 B). In my specimens the anterior half of the anterior median bone 

 (A) has a different shape from that given in Dr. Giinther's plate 

 xxxiv. fig. 1, and its margins are very well defined. Moreover, in 



