38 PROCEEDINGS OF THE NATIONAL, MUSEUM. vol.64. 



is short, bluntly rounded, as in the other species, and is situated 

 opposite the base of the inner of the two large prongs. The two 

 prongs of these hooks correspond to the middle and outer prongs of 

 the three-tined hooks of the three foregoing species. The rudiment 

 of a third, inner prong may be made out on close examination (fig. 

 43). 



Strobile. — The neck, in all specimens seen, is slightly enlarged next 

 the scolex, the enlarged portion being free from spines. Back of this 

 point the neck narrows for a short distance, then enlarges, and has a 

 tendency to enlarge and narrow irregularly. It is densely covered 

 with relatively large, straight spines with strong basal supports 

 which are elliptical in optical section. The neck is relatively long, 

 and the first segments to appear are squarish, but are quickly fol- 

 lowed by long and slender segments. Mature proglottides were not 

 seen. The posterior segments show a tendency to become relatively 

 broad, and are more or less rounded at the extremities. The genital 

 pore appears to be a little back of the middle of a margin. 



Measurements of four specimens mounted in balsam : 



Length of strobile _• 20.00 IG. 00 14.00 25.00 



Breadth of scolex, excluding hooks-^ 0. 46 0. 35 0. 42 0. 42 



Length of scolex 0.46 0.39 0.42 0.42 



Length of hook, maximum 0.11 0.12 0.10 0.12 



Distance to first distinct segment 0. 036 0. 036 0. 036 0. 036 



Length of posterior segments 0.77 1.29 0.76 1.33 



Breadth of posterior segments 0. 68 0. 49 0. 43 0. 28 



From spiral valve of Cestradon zygaeiia. 



7V^^— U.S.N.M., Helm. Coll. 7688. 



1911, July 5: 5, with many examples of PlatyhotJiri^vm 'parvum. 



1913, August 1:1. 



PLATYBOTHRIUM CERVINUM Linton. 



Plato 5. fig. 51. 



1890, pp. 820-823, pi. 8, figs. 8-10 ; 9, fig. 1. 

 1901, p. 427. 

 1911, p. 587. 



Hosts; Garcharhinus ohscwiios^ C. connniersonii^ C. niilherti, Galeus 

 gloMcus., and Scoliodon terrae-novae ; spiral valve. 



This genus was based on a single specimen from the spiral valve of 

 the dusky shark (Carchwhinus obscurus), collected at Woods Hole, 

 August 12, 1887. It has been found on but three occasions since that 

 date, all at Woods Hole. 



On September 12, 1912, I found a single specimen of this species 

 in a dusky shark. Dimensions, in balsam: Length 61; breadth of 

 scolex in front, 0.2, middle, 0.52, posterior end, 0.12; length of scolex, 

 0.40; length of hooks, 0.15; breadth of pair of hooks, 0.21; distance 



