42 PROCEEDINGS OF THE NATIONAL MUSEUM". vol. 64. 



1913, August 13: Numerous, large and small, in spiral valve, 

 largest 1 meter in length ; shark examined by Dr. G. A. MacCallum. 

 September 8: Numerous, large and small and free proglottides; 

 from a 150 centimeter shark. 



1914, August 12: Numerous, large, medium and small, and numer- 

 ous free proglottides from spiral valve. 



1915, July 31 : Many different sizes, small to adult, numerous free 

 proglottides, in spiral valve. August 5 : Many, large and small, and 

 great numbers of free proglottides in the chyle of the spiral valve. 



1919, August 19: Large nimibers, adult and young, and of free 

 proglottides collected by George M. Gray. 

 (U.S.N.M., Helm. Coll. 7691.) 



Cestracion Bygaena. 



1913, August 1 : 1, in spiral valve. Dimensions, in balsam: Length 

 32; scolex, length 0.21, breadth 0.18; breadth of pseudoscolex 1.47; 

 breadth of strobile behind pseudoscolex 1.05; posterior segment, 

 length 1.40, breadth 1.90. Dimensions in alcohol before mounting: 

 Length 40; diameter of scolex 0,22; diameter of pseudoscolex 1.82. 

 The scolex agrees in details of structure with that of the form from 

 the tiger shark, but is somewhat smaller. Thus, a scolex from the tiger 

 shark, in balsam, measures 0.26 in diameter, and 0.30 in length. The 

 chief difference between this example from the hammerhead and 

 typical specimens from the tiger shark is that of size. WliiJe the 

 posterior proglottides in the hammerhead specimen are not ripe, they 

 are sufficiently developed to show all the genitalia except the uterus, 

 the rudiment of which, however, can be distinguished in the posterior 

 segment. Segments can be distinguished about 3 mm. back of the 

 pseudoscolex, where their length is about 0.04, and their breadth 1.12. 

 Ten mm. back of the pseudoscolex the proglottides are 0.32 in length 

 and 1.54 in breadth. This is in sharp contrast with specimens from 

 the tiger shark, which may be a meter or more in length, the first 

 segments appearing at about one-third of the length of the strobile 

 back of the pseudoscolex. 



In spite of these differences, the details of structure are so similar 

 in these cestodes that I do not feel justified in referring those from 

 the hammerhead to a new species. It will be noted that while this 

 cestode is very abundant in the tiger shark, it has been found on but 

 three occasions in the hammerhead. It seems probable that the 

 difference, which is mainly that of size, is due to the fact that the tiger 

 shark furnishes a better physiological habitat for this particular 

 species than does the hammerhead. Confirmation of this view is 

 found in the condition of the material collected from a hammerhead 

 in 1914, noted below. 



The scolex was very active, projecting itself into an elongated, 

 conical, proboscis-like tip, then retracting until it was hidden in the 



