CATTDAL DIPLOSPONDTLT OF SHARKS. 49 



occurs in forms like Acantliias, Scymnus, and Mhina which are 

 destitute of the anal fin, the transitional region o£ the vertebral 

 column cannot bear any relation to this appendage. 



It is evidently to this transition region that Miiiler is referring, 

 when he writes (14. p. 156) : — " Bei Zygcena fand ich noch das 

 merkwlirdige, dass an einigen "Wirbeln des mittlern Theils der V/ir- 

 belsaule sogar 3 Bogenstiicke hinter einander auf einen Wirbel 

 jederseits kommen, wahrend die meisten Wirbel nur 2 Bogenpaare 

 haben." The regularity of the neural plates over the centra in 

 the tail-region was such that the duplicity of vertebrae passed 

 without notice, but the irregularity in the transition region 

 did not fail to catch the eye. Since Miiiler did not notice the 

 relations of the vertebrae to the neuromeres and myomeres, the 

 differences between the caudal and trunk vertebrae escaped him* 

 According to Mayer (13. p. 263), the much misunderstood state- 

 ment of Kolliker's with regard to Septanchios (vide postea, 

 p. 51 footnote) also refers to the few vertebrae in the transition 

 region. 



Gregenbaur (5. pi. 9. fig. 19) has figured a portion of the 

 vertebral column of Cestracion, from the vertebra bearing the 

 last rib to that with the fifth haemal arch. The figure, however, 

 shows no nerve foramina nor introduction of new intercrural 

 plates, but perfect regularity such as would occur in the trunk 

 region. Although, therefore, this is obviously the region of 

 transition, the irregularities which must have existed are not 

 shown. 



The fullest information on the subject is that furnished by 

 Mayer, whose illustrations (13. pis. 18 & 19) include the transi- 

 tional vertebrae of Scyllium, Mtistehis, Centrophorus, Heptanclius, 

 Scymmis, Acantliias, and Bliina. He roughly describes (13. 

 p. 266) the region in question in ScylUmn, but does not discuss 

 the detailed relations of the neural arches. He merely states 

 that the arches are irregular, and that the bodies may carry three 

 pairs of arches. 



In the specimen of ScylUum catuhts depicted in fig. 2, the irre- 

 gularities commence immediately behind the last rib-bearing 

 vertebra. The vertebra marked 2 has the crural plate {cr) ex- 

 ceptionally broad, and an additional neural spine {ns) is super- 

 posed upon it. This in itself is an exceptional occurrence, for 

 the neural spines normally lie over the boundary-lines between 

 the neural plates, and not directly over a plate. The third 



■LINN. JOTTRN. — ZOOLOGY, VOL. XXVTI. 4 



