1906.] THE SKULL OF A YOUNG RIBBON-FISH. 551 



The Interopercular is as much elongated as the preopercular ; 

 but is not so broad and also presents considerable difference from 

 that of R. glesne. In the latter the posterior margin is curved 

 sharply upwards and the dorsal edge is straight ; but in li. parkeri 

 the posteiior end of the bone is scarcely turned up and the upper 

 margin is excavated. The markings on this bone originate 

 almost exactly in the centre, whereas in R. glesne they are 

 described as originating "at the junction of the antei'ior and middle 

 thirds." 



D. The Craniwm. 



After the removal of the jaws and suspensorium, the form of 

 the cranium, as jseen from the side (PI. XXXIX. fig. 6), is some- 

 what like that of a bird's skull. Tliis appearance is due to its 

 length, to the large orbit, and to the beak-like prenasal rostrum. 

 On the dorsal surface are two median grooves, lying end to end, 

 meeting above the centre of the orbit (PI. XXXIX. fig. 4). The 

 anterior dorsal groove has as its floor a plate of cartilage named 

 the " tegmen cranii " by Parker, while its sides are formed by 

 the frontals. In this groove the nasal process (jmixJ) of the pre- 

 maxilla lies (PI. XXXIX. fig. 3). The posterior groove has as its 

 floor the supraoccipital and as its sides the epiotics and parietals : 

 it is scarcely existent in R. gles7ie. If the cranium of R. piarkeri 

 be compared with that of R. glesne, the most striking difi'erence is 

 the greater length and less vertical height in the present species. 

 This excess of length is mainly due to the greater development 

 of the preorbital region or " beak," which is nearly as long as 

 the orbital region, whereas in R. glesne (PI. XXXIX. fig. 7) it is 

 less than half this proportion. The length of the cranium in the 

 latter species is 1 1 times the greatest height, but the propoi'tion 

 of length to height in the present fish is 2:1. This difference in 

 relative length is due partly to the greater development of the 

 preorbital region, and in a small degree to the absence of the 

 "subcranial crest" described by Parker, formed by parasphenoid, 

 basi-occipital, and opisthotic. 



The Occipital Region (PI. XXXIX. figs. 4, 5, 6, 8). 



The Basi-occipital forms the greater part of the occipital condyle 

 (PL XXXIX. fig. 8), and is produced forwards and downwards as 

 a median ridge which extends anteriorly to meet the end of the 

 parasphenoid. In contrast to the condition in R. glesne, we may 

 note that it is the most posteriorly placed bone in the skull 

 (PI. XXXIX. fig. 6). Dorsally and anteriorly it is bounded by 

 the exoccipital and opisthothic, ventrally and anteriorly by the 

 parasphenoid. 



The ExoGcipitals entirely bound the foramen magnum 

 (PI. XXXIX. fig. 8), each meeting its fellow below it, so that each 

 takes a small share in the occipital condyle. The greater part of the 

 exoccipital is a posterior vertical plate which extends upwards to 

 meet the epiotic and outwards to meet the pterotic, while it sends 



