402 C. K. EASTMAN — JURASSIC FISHES FIIOIVI BLACK HILLS 



larged drawing of a detached scale. In no case has an3^thing heen added 

 to the drawings which was not clearly indicated on either the original 

 fossil or its counterpart, and the only chance for inaccuracy lies in the 

 delineation of facial sutures in plate 46. It is almost iin])ossible in some 

 cases to distinguish the latter from accidental fractures. To a limited 

 extent, therefore, this plate stands in the nature of a restoration. 



Details of Body Parts 



HEAD 



The cranial and facial bones are smooth and delicate, those of the 

 cranial roof forming a continuous shield, as shown in plate 46, figure 1. 

 The parietals can not be recognized on any of the specimens as separate 

 elements, and the median suture of the frontals is straight, making the 

 latter a symmetrical pair. The snout is. not produced, aiid the small 

 premaxillffi are not only mesially in contact, but appear to be fused (plate 

 47, figure 2 above the mandible). The maxillse are stout (plate 47, fig- 

 ure 3) and their convex oral margin provided with a row of minute teeth ; 

 the vomer and supramaxill?e have not been recognized as such. The 

 mandible is simple, rounded and expanded in its articulo-angular por- 

 tion (plate 46 and plate 47, figure 2). The orbits are large, with a pecu- 

 liar discoloration of the matrix in the center, possibly due to chemical 

 change of salts contained within the eye itself. Surrounding the orbits 

 is a ring of small, regularly disposed circumorbitals (plate 46, figure 1), 

 behind which are two moderate-sized but very thin postorbitals (sub- 

 orbitals), situated one above the other, the lowermost being somewhat 

 the larger. 



A narrow space intervenes behind the postorbitals, separating them 

 from the operculum and suboperculum, and extending downward and 

 forward as far as the mandible (plate 46, especially figure 1). This 

 space corresponds in position to the preoperculum and is remarkable 

 for its narrowness. The operculum considerably exceeds the suboper- 

 culum in size, is irregularly elliptical in shape, apparently convex in 

 some specimens (plate 46 and plate 47, figure 3), and in one at least is 

 , marked by faint radiating rugse (plate 46, figure 1). The suboperculum 

 is a triangular piece with rounded outlines, and encroaches over all the 

 space normall}'' occupied b}' the interoperculum. None of the specimens 

 show the latter plate as a separate element, and to all appearances the 

 series of branchiostegal ra3's follows immediately beneath the sub- and 

 preoperculum. There are about 14 laminar branchiostegal rays in the 

 series shown in plate 47, figure 2, and a lesser number are visible in 

 plate 46, figure 1. 



