58 ILLINOIS BIOLOGICAL MONOGRAPHS [58 



than in P. exiguus. The mass of coils of the vas deferens in P. pusillus 

 is much smaller than in P. exiguus and it lies almost wholly anterior 

 to the cirrus-pouch while in P. exiguus it lies more nearly at the end of 

 the cirrus-pouch. The testes are more numerous in P. pusillus and the 

 cirrus-pouch is much shorter than in P. exiguus. The lobes of the 

 ovaries in P. pusillus are bent strongly posteriad while such a condition 

 is never so marked in P. exiguus. The proportions of the proglottids 

 are greatly different. 



P. exiguus closely resembles P. fallax La Rue. It differs from that 

 species in having a smaller head, smaller suckers, a smaller fifth sucker, 

 a shorter neck, fewer and smaller proglottids, more testes, a shorter 

 cirrus when evaginated, and a greater number of lateral uterine out- 

 pocketings. The embryos of P. exiguus are smaller than in P. fallax. 

 The differences are of such a character and of such a constancy as to 

 be of specific importance. This species resembles P. agonis Barbieri 

 even more than it does P. fallax except that P. agonis has no well de- 

 veloped fifth sucker. In total length and in the size of the proglottids 

 P. exiguus is the larger. P. exiguus has more uterine pouches and 

 fewer but larger testes than has P. agonis. Unfortunately Barbieri 's 

 description fails in some particulars, nevertheless sufficient data are 

 given to establish the difference between the two species. P. exiguus 

 resembles P. clubius La Rue but its head, its suckers, its proglottids, 

 and its cirrus-pouch are smaller. These two species have about the 

 same number of testes. The embryo of P. dubius is much larger than 

 that of P. exiguus. P. exiguus differs radically from P. ambiguus and 

 P. esocis in the possession of a fifth sucker. Other points of difference 

 are also to be found. P. exiguus is smaller than other North American 

 species of Proteocephalus. 



PROTEOCEPHALUS PUSILLUS Ward 



[Figs. 2-4, 53-55] 



1910 : Proteocephalus pusillus Ward, 1910 :1185-1187 

 1911 : Proteocephalus pusillus La Rue, 1911 :475 



Specific Diagnosis: Characters of the genus. Cestodes small, 

 length 30-50 mm. Maximum breadth 0.350 mm. Proglottids few. Seg- 

 mentation distinct. Head spheroidal, frequently much contracted, 

 breadth about 0.300 mm., length about 0.260 mm. Four suckers placed 

 at broadest zone of head. Suckers deep, 0.140 mm. long by 0.110 mm. 

 broad. Fifth sucker well developed, muscular, 0.060 mm. in diameter. 

 Neck 1.0-1.5 mm. long by 0.21 mm. broad. First proglottids broader 

 than long, breadth of same 0.090 mm. Mature proglottids longer than 



