550 PROF, W. B. BENHAM AND MR. W. J. DUNBAR ON [May 15, 



The Interhyal, connecting the hyomandibular with the rest of 

 the hyoid arch, articulates with it almost at the same point as 

 the symplectic does. It is a small bone tipped at both ends with 

 cartilage. 



The rest of the hyoid arch is much compressed laterally so as 

 to form the " hyoidean cornu " of Prof. Parker, and is made up of 

 four bones, the epi- cerato- and two hypo-hyals. Of these the 

 Epihycd, a flat semicircular bone, articulates with the interhyal, 

 forming the upper rounded end of the cornu. It lies posteriorly 

 to the ceratohyal as in R. glesne. 



The Ceratohyal, like that of R. glesne, is the largest of these 

 bones, but relatively much longer and narrower than in that 

 species. It intervenes between the epihyal and the two hypohyals, 

 forming the posterior lower margin and but a small part of the 

 dorsal margin of the arch. 



The Hypohyals, of which there are two, take a greater share in 

 the formation of the " hyoidean cornu " than in R. glesne, being 

 together almost equal to the ceratohyal. The larger and upper 

 one forms most of the anterior margin of the cornu, while the 

 smaller and lower one {h.hy.') forms the rest of the anterior and 

 the whole of the mesial margin where the hyoidean cornu is 

 a,ttached to the glossohyal. This lower one seems forked on the 

 inner side, but this is due to the lower corner of the ceratohyal 

 overlapping and concealing part of it. In fig. 2 the concealed 

 part of the margin is indicated by a dotted line. 



C. The Opercular Bones. 



Of these there are four, and all are visible through the skin of 

 uninjured fish. 



The Opercxdar itself is a subcircular bone having a concave 

 facet for articulation with the hyomandibular. It has three 

 borders, all curved, one of which faces anteriorly, one dorsally, 

 and the other postei-o-ventrally. Below the opercular is the 

 Suhopercular, a narrow plate of very delicate bone. In neither 

 of these bones is there a marked difference from the corresponding 

 bones in R. glesne (PI. XXXIX. figs. 6 & 7). 



The Preopercular is one of the largest bones in the skull, 

 extending from the anterior border of the opercular to the 

 anterior end of the quadrate, making a total length of one and 

 three-quarter inches. Thus it forms a considerable part of the 

 suborbital region of the face. Instead of the greatest length 

 being in a vertical direction as is usual, it is here in a horizontal 

 direction. The difference between the preopercular in the two 

 species is very marked. In the present species the greater part 

 of the bone is horizontal and suborbital, whereas in R. glesne the 

 greater part is vertical and postorbital. Again, in the latter 

 species the anterior and posterior margins are gently and regularly 

 curved, but in R. parkeri the lower margin is straight for some 

 distance and then curves suddenly upwards (PL XXXVIII, fig. 1), 



