THE ELASMOBRANCH FISHES 55 



rudiments by growth extend medially, forming the floor of the cranium; the 

 tral)eeulae bend to form the basal angle, and then project forward in the re- 

 gion of the rostrum, giving rise to the rostral plate. The alisphenoidals pro- 

 duce a large part of the sides of the cranium and extend dorsally, to form the 

 roof of the cranium. To these three pairs of cartilages the sense capsules fuse. 

 The nasal capsule joins the trabecula (rostral) ; the otic or auditory capsule 

 joins the parachordal; while the optic capsules, when present, remain free. 



In the adult sharks alid rays, as we shall see, the extent to wdiich the plates 

 increase in size and the modifications to which they are subject vary greatly. 

 Indeed, in various species of the same group these differences often produce 

 entirely unlike crania. An example of this variation may be seen in Zygaena, 

 the hammerhead (fig. 65), in which the cranium in the region of the eye is so 

 modified as to be unlike that of the nearlj^ related Gale us. The modification, or, 

 better, the progressive develo])ment of the cranium is best understood by con- 

 sidering first the simpler and more generalized, and then the more highly 

 specialized Elasmobranchs. 



In dorsal view the adult Elasmobranch cranium may be said to take the 

 shape of an hourglass (figs. 59, 61, 62). In this hourglass the basal segment is 

 formed by the enlarged otic or auditory capsules; the middle part of the in- 

 dentation contains the orbits for the eyes; and the top segment is produced by 

 the olfactory capsules. Upon this as an apex may arise longer or shorter rostral 

 cartilages {rs.). 



In more generalized forms, as, for example, Hcptanchus or Chlamydosel- 

 achus, the positions of the semicircular canals of the ear are evident as super- 

 ficial ridges on the surface of the auditory capsule. 



The ridge for the posterior canal runs from the parietal fossa or pit outward 

 and backward to the foramen for the ninth cranial nerve, while the ridge for 

 the anterior canal rises at the parietal fossa and runs forward at right angles 

 to the posterior canal. In more highly specialized types, w^here the parietal 

 fossa is shallower, external evidence of the canals is less distinct (fig. 62). 



From the bottom of the parietal fossa certain apertures lead to the ear. Two 

 of these are for the endolymphatic ducts {e.d., fig. 59) and two other accessory 

 apertures are the fenestrae (/».), for the perilymphatic spaces. In some forms 

 the parietal pit may be deep and the endolymphatic foramina may be sepa- 

 rated only by a short space, as in Heptanchus. Again it may be shallow, where- 

 upon these foramina are farther apart (8cylliu)}i). In still other forms, the 

 pit is only a slight depression and the endolymphatic foramina are more 

 widely removed from each other {Rhinobafis, fig. 62; Raja clavata; Trygon; 

 Myliohatis) . The fenestrae are apertures of much larger size than are the 

 foramina for the endolymphatic ducts, and have at times been confused with 

 them. In a type like Heterodontus (fig. 61) the parietal pit is deep and hence 

 the fenestrae are difficult to see. They are evident, however, in Pseudotriacis 

 (fn., fig. 59) and in Khinohatis (fig. 62). In a specimen of Galeorhmus, six 

 feet in length, and in a large Myliohatis calif ornicus, they were relatively of 

 immense size. 



