98 STUDIES ON THE ELASMOBRANCH SKELETON, 



exception of the last two or three, with a short basal element. In 

 general structure these fins bears a close i-esemblance to the 

 corresponding parts in (Jinglymostoma (Mivart, I.e., p. 446, pi. 

 LXXVT., fig. 1), except that the rudimentary basal plate is not 

 represented. The anal fin is smaller than the second dorsal, with 

 fifteen rays of similar shape to those of the dorsals. 



There are fifty-five pairs of well-developed ribs. Of these a 

 good many are divided into two segments. 



The caudal portion of the vertebral column is but slightly 

 curved. Its neural rays, about forty-five in number, are nearly 

 twice the length of the haemal rays. 



Of the peculiarities in the structure of the skeleton of Crosso- 

 rliinus, the most noteworthy are the form of the articular cavity for 

 the hyo-mandibular, the abnormal shape of the rostrum, the absence 

 of the external branchial arches, and the rigid nature of the 

 ventral portion of the pectoral arch. These, besides other less 

 important characteristics, definitely mark off the genus from the 

 Scyllidce with which it was previously placed. A characteristic 

 feature of the skeleton of Crossorhinus is the presence throughout 

 all the cartilages of innumerable small bony centres, giving the 

 skeleton a remarkably hard and rigid texture. 



PRISTIOPHORUS CIRRATUS. 

 Plate I., Pigs. 9-12. 



Skull and Visceral Arches. (Plate I., fig. 9.) 



In general form the skull, were the rostrum removed, would 

 not be unlike that of Heterodontus. The plane of the occipital 

 foramen is vertical. The surfaces of articulation for the hyo- 

 mandibular are antero-posteriorly elongated and placed low down 

 near the base, as in Rays. The auditory apertures are placed close 

 together near the middle line, and are placed at the bottom of a 

 deep excavation which is continued in front into a short shallow 



