ISISTIUS BRASILIENSIS. 35 



half as wide as the spiraelo, above the level of the base of the pectoral, not 

 in a groove. 



The Skull. As might bo expected from external resemblances, there is 

 much evidence of affinity in the skeletons of Jsistius and Sc)jmnorJu)tus, 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 Scynniorhinus. The skull, as seen from above, Plate II., 

 fig. 3, 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 II., figs. 3 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 suggestive 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 

 Scymnorhinus the mandibles are very massive, and together they are so 

 much wider than the skull that the hyomandibular Hes 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 II., 

 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 



