T^NIOID CESTODES OP NOKTH AMERICAN BIEDS. 55 



21. Scolex without rostellum, or with riidiuieiitary unarmed rostellum 49. 



Scolex with armed rostellum 22. 



22. Rostellum armed with a double crown of large hooks ; genital pores irregu- 



larly alternate ; uterus with median stem and lateral branches ; egg with 

 thin outer membrane and thick, brown, radially striated inner shell. 



Twnia, p. 100. 

 Egg with thin transparent shells 23. 



23. Para-uterine organ present ; rostellum armed with a double crown of 



hooks : 24. 



Para-uterine organ absent 26. 



24. Uterus single 2.5. 



Uterus more or less completely divided into two spherical sacs ; hooks of 



rostellum triangular Biuterina, p. 88. 



25. Uterus much broader than long ; para-uterine organ a transversely elongated 



parenchymatous mass in front of uterus ; on pore side of segment the large 

 usually ventral longitudinal excretory vessel is dorsal in position, and the 



• narrow usually dorsal vessel ventral, the position being normal on other 

 side 1 Culcitella, p. 86. 



Uterus slightly broader than long, crescentic, in front of ovary, or straight, 

 near posterior border of segment; para-uterine organ elongated antero- 

 posteriorly in front of uterus; position of excretory vessels normal on 

 both sides of segment Pariiterina, p. 85 



26. Rostellum armed with a single or double row of hooks; suckers armed or 



unarmed ■- 27. 



Rostellum armed with several rings of rose-thorn booklets ; a double set of 

 reproductive organs in each segment: two genital pores in each segment, 

 one on each side ; uterus reticular, later breaking up into egg capsules. 



Dipylidium, p. 84. 



27. A single set of reproductive organs in each segment 28. 



A double set of male reproductive organs and a single set of female organs 



with' two vaginae in each segment ; two genital pores in each segment, one 

 on each side; uterus sac-like, persistent Diploposthe, p. 101. 



28. Uterus persistent 29. 



Uterus breaks down into egg capsules, each containing one or several eggs ; 



rostellum armed with a double or single crown of hooks ; genital pores 

 irregularly alternate, rarely unilateral; testicles numerous, behind the 

 female glands or also laterally on both sides of the latter. 



^Jonopylidium, p. 76. 



29. Hooks of rostellum arranged in a circular crown 30. 



Hooks of rostellum arranged in a zigzag crown ; genital canals pass dorsal 



of excretory vessels; uterus very irregularly lobulated Angularia, p. 83. 



30. Crown of hooks single 31. 



Crown of hooks double 41. 



31. Genital pores unilateral 32. 



Genital pores alternate 34. 



32. Genital pores subdorsal ; testicles few, but more than 4 in each segment ; 



eggs few TricJiocephaloicl<^s, p. 73. 



Genital pores strictly marginal ; testicles numerous or few ; eggs numerous, 

 or. rarely, fen 33. 



33. Base of cirrus provided with one or two pairs of powerful spines lying-in 



special pockets ; genital canals pass between longitudinal excretory 

 vessels ■--Gryporhynclius, p. 83. 



