1876.] PROF. T. H. HUXLEY ON CERATODUS FORSTERI. 33 



palato-quadrate cartilage just above the middle of the palatine tooth. 

 It thence descends with an outward convexity and inward concavity, 

 and terminates in the upper lip near the angle of the mouth. 



Dr. Giinther states (l. c. p. 524) that " the body of the mandible 

 is persistent cartilage ; but its entire outer and inner surfaces are 



covered by bone, forming an articular and a dentary piece 



The articular and dentary bones meet near the top of a low but strong 

 coronoid process, and again at the syjnphysis, which is formed by 

 fibrous tissue, and may easily be separated by the knife .... In 

 front of the jaw the cartilage is expanded into a slightly concave 

 lamella (lower labial cartilagej." 



Cerafodus forsteri. Underview of tbe skull, showing the vomerine teeth ( V.t), 

 the palatine teeth, the mesethmoid cartilage (M.E), and the upper labial 

 cartilages (1, 2) in place. The dotted lines An, p.n indicate the form and 

 position of the anterior and the posterior nares. 



I find a persistent Meckelian cartilage, such as that here described ; 

 but as, after careful removal of the ensheathing bones, I have been 

 unable to discover any separation between this lamellar expansion 

 and the rest of the cartilage, I am in doubt whether the lamella 

 represents the lower labial cartilage or not. The analogy of the 

 Frog, however, leads one to suspect that distinct lower labial carti- 

 lages may exist in the young Ceratodus. 



Dr. Giinther does not mention a third ensheathing bone (figs. 2 and 

 4, D) which is united by suture with the other two, and lies on each 

 side of the symphysis on the ventral face of the mandible. It is a flat 

 plate, of a triangular form, with a thick rugose inner edge for the 

 attachment of the symphysial ligament. Its posterior edge is thin and 

 concave ; its external edge is also thin and overlaps the bone termed 

 "articular" by Dr. Giinther, uniting with it by a squamous suture. 

 The outer half of its dorsal aspect is smooth, and helps to support the 

 ventral face of Meckel's cartilage ; the inner or symphysial half pre- 

 sents a broad rough triangular surface, which extends on the inner 



Proc. ZooL. Soc— 187G, No. III. 3 



