1887.] TELEOSTEAN GENUS RHACOLEPIS. 537 



maxillae, and dentaries are likewise armed with a moderately powerful 

 series. These are elongated cones, closely approximated, and varying 

 but little in size, though those on the palatines are apparently the 

 largest. The cleft of the mouth is slightly turned upwards, and the 

 lower jaw scarcely projects beyond the upper ; and of the two 

 elements entering into the bony margin above, the maxillae have 

 much the greater extent. 



But the most striking feature in the head is presented by the ring 

 of circumorbital bones, which attain to an extraordinary size (Plate 

 XLVI. figs. 1. 2, 5 ; Plate XLVII. figs. 4, 5). Two, or perhaps 

 three, of considerable dimensions are situated behind the eye, while 

 the largest occupies a postero-inferior position ; and the anteriorly 

 directed process of the latter, which is very narrow in R. buccalis, 

 bounds the orbit below, in conjunction with the small foremost 

 element of the ring. Posterior to this series, the long narrow 

 preoperculum is seen ; and still beyond are the other elements of the 

 opercular fold. The operculum itself varies in form in the different 

 species (compare Plate XLVI. figs. 1, 2 ; Plate XLVII. figs. 4, .5) ; 

 the suboperculum is relatively large ; and there is a distinct triangular 

 interoperculum. None of these bones exhibit any definite ornament, 

 sculpturing, or marginal denticulation, and there was no extension of 

 the scales over any part of the cephalic region. The branchiostegal 

 rays attached to the epihyal are about ten in number, and large and 

 much expanded ; but in front of these the ceratohyal supports an 

 apparently equal series, which are quite of small size (Plate XLVII. 

 fig. 1), and Hkewise differ in being considerably "spaced out." 



The vertebral column is not completely shown in any specimen, 

 and it is thus only possible to determine the number of vertebrae 

 approximately : in R. buccalis there appear to be not less than 24 

 in the abdominal region, and perhaps as many as 20 in the caudal. 

 The centra are well ossified, though perforated in the middle for the 

 passage of a remnant of the notochord ; and they exhibit no large 

 lateral excavations, but are marked by delicate longitudinally 

 extended pittings (Plate XLVI. fig. 4 a). The ribs are tolerably 

 massive, as shown by B. M. no. 47900, and the same fossil displays 

 a considerable number of crushed intermuscular bones. Both neural 

 and haemal arches in the caudal region are remarkably strong and 

 elongate; but it is unfortunately impossible to determine with certainty 

 the modifications for the support of the tail-fin. 



In the pectoral arch the form of the clavicle is well shown, 

 especially by one small fragment (no. P 1958 c). It has a gentle 

 longitudinal curve, the concavity being anterior ; and there is a 

 comparatively broad, inwardly directed wing, in addition to the 

 exposed part in the plane of the side of the body. The pectoral Jin 

 is robvist (Plate XLVI. fig. 1), and the several stout rays are divided 

 distally ; but it is impossible to determine the exact number of these 

 rays, though there cannot have been less than 18 or 20 in 

 R. buccalis. 



The '^pelvic" bones are only well shown in one specimen, belonging 

 to the small species just mentioned, and the element of the right side 



