§^ bit. %. G. RIDEWOOD OJr TfiE 



variable length of the chorda centra which causes the discre- 

 pancies, he proceeds : — " These apparent irregularities reach 

 their climax in the tail of many Elasmobrauchs, where exactly 

 the reverse takes place to what occurs in the trunk, in this way, 

 that the chorda centra are so numerous, or so short, that two of 

 them fall to the share of one true segment. The number of 

 dorsal cartilaginous pieces varies extremely." Now, as a matter 

 of fact, the neural plates are arranged with the greatest regu- 

 larity, as will be seen by a glauce at fig. 1 ; namely, one plate 

 united with each centrum and one plate intercalated, the median 

 dorsal cartilages being regularly disposed over the intervals. In 

 fact Mayer (whose paper is quoted by Gradow on p. 196) had 

 already written (13. p. 266), " Nur die an der TJbergaagsstelle 

 befindlichen Wirbel zeigen allerlei Unregelmassigkeiteu m Lage 

 und Anzahl jener Stiieke." The only shark in which I have been 

 able to detect any want of correspondence in the tail region 

 between the neural plates and the centra is G-aleus. Since, as a 

 rule, the elements of the caudal vertebrae are as regularly dis- 

 posed as are those of the trunk vertebrae, the " explanation " 

 cannot be considered valid. 



Dr. Gadow further observes (4. p. 195) that the " intercalatiun 

 or wedging-in of these various cartilages can be followed from 

 the simplest to the most complicated conditions iu the Rajidse." 

 It is to be regretted that he does not give illustrations of these, 

 for he acknowledges that Hasse's figures do not explain the facts. 

 But it must here be pointed out that the Rays are less primitive 

 than the Sharks ; and that the fact of the phenomenon being 



Streifen, welcher der Mitte der Baseu der Neur- und Haemapophysen entspricht 

 und senkrecht verlaufend die iiberrascbendste Aehnlichteit mit einein Zwischeii- 

 wirbelgewebe hat." And one year previously to this von Ibering (9. p. 222) 

 had suggested that Kollikei''s error arose from his counting the neural spines 

 as arches. The figure, moreover, which Mayer has given of Heptanchus (13. 

 pi. 18. fig. 10) sliows the usual Selachian diploapondyly, commencing on the 

 fifty-ninth vertebra. The deceptive bands on the centra mentioned by Hasse 

 are clearlj^ shown, but there is nothing abnormal in the regularity of the neural 

 arches. In the accompanying text (13. p. 263) Mayer shows tliat the statement 

 of XoUiker's is correct if read as referring only to the few transitional vertebras 

 between the trunk and tail regions. But none of these explanations can apply 

 to Dr. Gadow's assertion (4. p. 194) that " each long centrum actually belongs to 

 two true segments " — a statement which cannot refer to the alternation of the 

 intercentral plates with those of the intermuscular septa, for that is normal in 

 Sharks, and would not be worth mentioning. 



