450 ALDEN B. DAWSON 



medially into the ventral side of the cloaca, almost opposite the 

 more dorsal urogenital papillae (figs. 4, 5, 8, ur.hl.o.). 



Caudad to the orifice of the urinary bladder a prominent longi- 

 tudinal fold projects from the mid ventral wall, and on either side 

 of it other smaller irregular folds can be distinguished (fig. 5, 

 mv.J., V.J.). The main fold continues caudally for a short dis- 

 tance as a single fold, but soon becomes doubled and is eventually 

 broken up into the thin ridges which cover the walls of the ven- 

 tral trough (figs. 5, 9, 10, v.tr., vs.). Laterally the wall of the 

 cloacal tube is also modified by two low folds which extend, on 

 either side, from the regions of the urogenital papillae caudally 

 to the cephalic ends of the longitudinal lateral furrows, with 

 whose dorsal walls they merge (figs. 4, 9, It.f.). Furthermore, 

 when the cavity of the cloaca is laid open by a longitudinal ven- 

 tral incision so that the dorsal portion of the cloacal tube is ex- 

 posed, the lateral folds, with the aid of the median dorsal groove 

 and lateral furrows, are seen to mark off a Y-shaped area, the 

 stem of which extends cephalad toward the region of the urogeni- 

 tal papillae (fig. 4, Y). 



Another striking feature of the cephalic portion of the cloacal 

 tube is the presence of large numbers of melanophores in the 

 underlying connective tissue. No other portion of the cloacal 

 cavity exhibits a like pigmentation, although a few scattered 

 melanophores can occasionally be seen in other regions. 



The ventral trough, longitudinal lateral folds, and median 

 dorsal groove already referred to, are found in the more caudal 

 portion of the cloacal tube. The ridges of the ventral trough 

 are relatively high and thin. They run almost parallel, but 

 diverge slightly as they approach the cloacal chamber. The 

 number of ridges present is quite constant, the average being 

 thirty-two, although thirty-four ridges can occasionally be 

 counted (figs. 13, 14, v.r.). The longitudinal lateral furrows 

 and median dorsal groove do not exhibit any conspicuous modi- 

 fications and, gradually growing shallower as they pass cau- 

 dally, are eventually obliterated in the region of the cloacal 

 chamber. 



