1910.] ANATOMY OF THE FRILLED SHARK. 551 



suspension of the jaws is amphistylie, and the palatoquadrate 

 cartilages have a postorbital articulation with the cranium. 

 Moreover, the extension of the jaws posterior to the cranium is 

 but very slight. 



IV. The Membranous Labyrinth. (Plate XLIII. figs. 7 & 8.) 



The organ of the right side of the head has been worked out 

 by dissecting away the surrounding cartilage, and forms the basis 

 of the following account. 



On removing the skin from the dorsal surface of the cranium 

 it is seen that the parietal fossa is rather deep and possesses four 

 apertures, two on either side of the median longitudinal line. 

 One of these apertures, the anterior, is small, and transmits the 

 ductus endolymphaticus. The posterior is larger and is closed 

 with soft subcutaneous tissue. It is an opening into the peri- 

 lymph cavity surrounding the posterior vertical canal, and seems 

 to correspond to the tympanic aperture which Howes (19) 

 described in Raia> Before proceeding further, I may mention 

 that in this account I am following the nomenclature used by 

 Htewart (30), which differs somewhat from that used by Retzius 

 (28) in his great monograph. 



The ductus endolymphaticus, on emerging from its cranial 

 foramen, soon expands into the saccus endolymphaticus. The 

 latter lies partly in the parietal fossa and is partly attached to 

 the under surface of the skin covering this region. It is fairly 

 regular in sha,pe, somewhat rounded on its anterior surface, and 

 extends posteriorly in a slightly outward direction, gradually 

 becoming attenuated until it reaches its external aperture, which 

 is quite small. Internally the ductus endolymphaticus leads into 

 the saccidus. This is not rounded, but is laterally flattened, and 

 gives off' at its postero-inferior end the lagena in the form of a 

 simple cascum. 



The utricuhis in this species is like that in other Elasmo- 

 branchs, being divided into two portions, anterior and posterior, 

 which do not communicate directly with each other, but in- 

 directly through the sacculus. 



The anterior utricle is rather laterally compressed and gives 

 off the anterior canal dorsally. The latter curves forward and 

 slightly outward, and describes almost a semicircle in its course, 

 expanding at its lower end into the anterior ampidla, which then 

 opens by a wide portion into the lower end of the utricle again. 



The recessus utricidi is a somewhat spherical structure on the 

 inferior and outer border of the anterior utricle. It communi- 

 cates with the latter by means of a slit-like aperture just below 

 that leading into the ampidla externus. The anterior utricle 

 does not open directly into the sacculus, but communicates in- 

 directly with it through the recessus utriculi, which opens into 

 the saccidus by means of a rounded aperture on the postero- 

 dorsal side of the recessus. 



