MUSEUM OF COMPARATIVE ZOOLOGY. 205 



( c in upper quarter. 



\a in the middle third but above the middle. 

 From tlie middle region the chorda and aorta both rise until each 

 breaks through the surface of the cartilage, the chorda to end in the cal- 

 careous incrustation of the cartilage, and the aorta to enter the pituitary 

 space from the summit of the saddleback. Two sections from region 

 ( 3 ) gave in 



(■ c on the upper surface of cartilage. 



Section No. 1" -s • ^ - , ,r 



la in upper quarter of upper half. 



« « 0" 5 ^ section in front of chorda end. 



( a emerges from top of saddleback. 



It will be seen from this table that the chorda occupies in the occipi- 

 tal region the middle of the cartilaginous plate of ttie cranial floor, 

 and that from the end of the cone-shaped body, which was in the indi- 

 vidual dissected about 1 cm. long, the thread remnant rises gradually 

 but continuously until it reaches the inner surface of the floor of the 

 cranium. This point, as the sections show, was behind the apex of the 

 pituitary eminence (Sattellehne). Further, we find that the course of 

 the aorta remnant is very nearly parallel with that of the chorda, and 

 that it issues from the apex of the pituitary eminence in exactly the man- 

 ner described by Gegenbaur for the chorda of other Elasmobranchs. 



Gegenbaur " has investigated the subject of the chorda termination 

 very thoroughly, and judging from the text and plates mostly by means 

 of longitudinal section. There are several figures of cross sections of 

 the basis cranii, showing the structure which he calls " Chorda," but 

 which resembles the more ventrally placed aorta as I find it in Chlamy- 

 doselachus. As the author says, it is almost always a very difficult 

 matter to determine the exact position and manner of the chorda ter- 

 mination in adult animals ; much easier with young animals or older 

 embryos. The author studied, among others, the genera Acanthias, 

 Heptanchus, and Centrophorus. He says (loc. cit., p. 121): "Die 

 Chorda tritt mit ihrer Aufwartskriimmung immer naher an die in- 

 uenflache der Schiidelbasis und steigt dabei in der Sattellehne em- 

 por, welche sie dicht unter deren hinterer Flache durchsetzt, um niihe 

 an der Kante dieses Yorsprunges unter das Perichondrium zu treten. 

 Wo die Sattellehne starke corticale Verkalkungen zeigt ist das zuc^e- 

 spitzte Ende der Chorda noch in diese eingebetet. Fig. 7, Taf. XIV. 

 gibt eine Darstellung dieses Verhaltens in einem 22 cm. langen 

 Acanthias Embrj-o auf dem Medianschnitte. Beim ausgewachsenen 



1" Gegenbaur, C. Das Kopfskelet der Selachier. Leipzig, 1872. 



