482 Illinois State Lahoratorij of Natural History. 



recognized, and under some conditions are very deep. The 

 mouth is relatively small, and placed far forward on a median 

 thickening of the rim of the sucker beneath the preocular 

 lobe. 



There is no distinct clitellum, and the gonopores have the 

 usual glossiphonid position of XI/XII for the male and 

 XII a '2ja 3 for the female. The nephridial openings, of 

 which there are 16 pairs, are situated on low papilhie a little 

 anterior to the middle of the annulus a 2 on somites YIII to 

 XXIII. 



Compared with P. riigosa the posterior sucker is relatively 

 large, its anterior margin reaching to XXII and the posterior 

 largely free. It is circular, flat, and smooth. The anus is 

 large and at XXVII/XXVIII. 



Annuli and Somites (PI. XLIL, Fig. 1). — The annulation of 

 this species is especially interesting and suggestive, inasmuch 

 as it i)resents the strongest kind of confirmation of the com- 

 parison recently made (Moore, 1900) between the biannulate 

 somite of MlcrohdcUa and the triannulate of the glossiphon- 

 ids. Even in the largest specimens, and more or less 

 obviously in all parts of the body, the annuli a 1 and a 2 are 

 evidently more closely associated than are ^2 and a'd. In 

 young examples and toward the extremities of the body, the 

 neuromeric limits of the somites and the more primitive 

 relations of their component annuli become extremely clear. 

 The gradations between the biannulate and the triannulate 

 types are so gentle that the exact number of complete somites 

 cannot be definitely stated; it is impossible to decide just 

 where the biannulate type ends and the triannulate begins. 

 Of course this is more or less true of many leeches, but one 

 can usually rest satisfied with a decision that a particular 

 furrow is to be described as incipient or as complete. Not so, 

 however, with this species. The different values of these fur- 

 rows have, however, not been brought out in the plate. 



I and II are preocular and are rarely separated by a recog- 

 nizable furrow. In some cases the furrow II/III is very 

 faint. At least one pair of metameric sensillse are constantly 



