ISISTIUS BRASILIENSIS. By 
half as wide as the spiracle, above the level of the base of the pectoral, not 
in a groove. 
The Skull. As might be expected from external resemblances, there is 
much evidence of affinity in the skeletons of Jsistivs and Scymunorhinus, yet 
differentiation has carried Isistius the farther from all the other sharks, as 
may readily be seen in comparisons of the features in which it differs most 
from the species of Scymnorhinus. The skull, as seen from above, Plate IL., 
fie. 8, is narrow and elongate, the width across the nasal capsules is about 
equal to that across the postorbital processes, or to that across the occiput. 
A distinguishing feature of prominence is to be seen in the rostral cartilage, 
which is reduced to a slender rod, rounded at the forward end, slightly 
attached to the cartilages below it between the nasal capsules, and tapering 
backward to a slight, possibly ligamentous attachment above the prefrontal 
foramen, Plate IL, figs. 8 and 5. The openings for the passage of the eth- 
moidal canal and the ophthalmic branch are rather close together in a 
depression, and the supraorbital openings, so noticeable on other sharks, are 
minute or invisible. The aqueducts of the vestibule are shown in fig. 5; 
in fig. 3 they are hidden by projecting cartilage. The cranial chamber is 
deepest backward, behind the pituitary fossa; it loses depth rapidly forward 
and is sugyestive of a comparatively greater development of the hind brain. 
Between the orbits the lower portion of the skull is narrowly compressed. 
The section from which fig. 5 of Plate II. was drawn was cut a little to the 
right of the middle; this has left intact the blade-like portion between the 
orbits, bounded anteriorly in the figure by the cut surface behind the nasal 
capsule, and posteriorly by that below the pituitary cavity, the line from 
this last to the lower surface being an accidental result of drying. As in 
Seymnorhinus the mandibles are very massive, and together they are so 
much wider than the skull that the hyomandibular lies transversely, with 
the end to which the lower jaws and the ceratohyal are attached higher 
than that attached to the skull. In normal position, what in fig. 4, Plate IT., 
is the lower end of the hyomandibular rests against ceratohyal and mecke- 
lian, with its anterior angle against a solid, inward-directed process of the 
latter, that from the side, in fig. 4, presents the appearance of a separate 
cartilage. The upper jaws, quadrato-pterygoids, are compressed, blade-like, 
and twisted. At the symphysis the narrow lower edge of the skull rests in 
a deep notch between them. The teeth are situated on the hinder side of 
the lower edge. At the point of attachment to the lower jaw, immediately 
