AET. 21. PARASITES OF SHARKS AND SKATES LINTON. 31 



The finding of another species in the dusky shark, and of two 

 additional species in the hammerhead shark {Gestracion zygaena) 

 makes a restatement of the generic characters desirable. 



Characters of genus. — Four cruciformly placed bothria, each with 

 an auxiliaiy acetabular space in fi*ont of a pair of compound hooks, 

 and loculate at the posterior end. 



In some contraction states the bothria appear to be tubular, thus 

 suggesting, as was stated in the original description of the genus, 

 Diesing's Cylindrophorus typicus, a name which he proposed for 

 Wagener's Tetrabothmum carcharia£-7'ondoletti. Further, in some 

 contraction states of the strobile, especially noticeable in the species 

 from the hammerhead shark, the neck bears a strong resemblance to 

 Wagener's figure.^^ 



PHOREIOBOTHRIUM LASIUM Linton. 



Plates 4 and 5, figs. 36-40 and 54. 



18896, pp. 474-476, pi. 4, figs. 24:-29. 

 1890, pp. 819-820. 

 1897a, p. 447. 



1900, pp. 272-273. 



1901, pp. 426, 427, 428. 

 1905, pp. 340-343. 

 1907e, p. 116. 



1908», p. 114. 



1908&, pp. 163, 165, 174. 



1911, p. 587. 



The cestodes recorded as P. lasium, from Gestracion zygaena ( 1900, 

 p. 273 ; 1901, p. 428) should be referred to P. pectinatum. 



Scolex. — This is somewhat rectaiigular in outline; the bothria, be- 

 ing provided with a thick muscular border, are rather rigid ; at rest 

 they are parallel with the neck, or slightly flaring at the posterior 

 end, in which case the scolex becomes somewhat hour-glass shape. 

 There is a short lappet-like projection at the back of each bothrium 

 near the posterior end, and the space thus formed is divided into a 

 considerable number, 20 or more, loculi (figs. 36, 38) . 



The muscular borders of the bothria are earned forward and f onn 

 a single acetabular space in front of the hooks. In some cases this 

 space appears to have a faint trilocular structure. Each bothrimn 

 carries a pair of three-pronged hooks. Each hook is like a short- 

 handled three-toothed rake, in which the oblong, bluntly rounded 

 embedded portion is the handle, a transverse bar is the head, and the 

 three prongs, which are nearly at right angles to the transverse bar 

 and are strongly reflected, are the teeth. The middle prong is about 

 twice as long as the inner and a little longer than the outer prong. 



18 Entwickelung der Cestoden, fig. 270. 



20:1 88— 25— Proc.N.M.vol.64 35 



