32G PROCEEDINGS OF THE NATIONAL MUSEUM vol. ii4 



four similar dentations, the paramedians a little larger than the 

 laterals. In lateral aspect the dorsal jaw averages 6.1 percent of the 

 total head length, with variation from 4.8-7.2 percent. 



Body of normal proportions for the genus, segment i somewhat 

 broader than head diameter, the body increasing gradually back to 

 segment vi and vii, thence tapering rather abruptly to x which is 

 distinctly narrower than the caudal sucker. Latter fairly large, 

 either equally (rarely) or slightly exceeding (usually) the greatest 

 head diameter. 



Male reproductive system (fig. 29) fairly large in its relative size, 

 occupying most of one side of segment vi. Bursa pyriform or sub- 

 cordate in shape, 1.5 times as long as broad or even less. Atrial 

 portion shorter than penial sheath. Ejaculatory duct short and 

 narrow, one-half to two-thu'ds as long as bursa and about a fourth as 

 wide. Spermiducal gland very long and abruptly refiexed at the 

 midlength, the ental half lying beside and against the ectal with the 

 ental end overlapping the bursa. No enlargement or lobation of the 

 gland at points of entry of the very slender deferent ducts. Prostate 

 relatively short and slender, less than two-thirds as long as spermi- 

 ducal gland, and about three-fourths its diameter, with a well-defined 

 terminal bulb. 



Spermatheca elongate and slender, curving dorsolaterad around the 

 gut in segment v and attaining the middorsal interior wall of the 

 segment. Basal two-thirds composed of the parallel-sided and slender 

 spermathecal duct, the ental third of the globose or fusiform sper- 

 mathecal bulb. No ental glandular process present. 



Variation. — A certain amount of variation affecting the relative 

 size and proportions of the sexual organs was detected, but none of 

 it appears to be associated with any sort of morphocline. A possible 

 geogi'aphic gradient may affect the jaw size — specimens from Kansas 

 have appreciably larger jaws which measure from 6-8 percent of the 

 head length, whereas worms from Wisconsin and South Dakota (some- 

 what larger in total size) have jaws with a length only 5 percent that 

 of the head. Presumably material from the intervening region will 

 be found to be intermediate in this respect. The shape of the jaws is 

 not affected by this variation in size. A summary of selected measure- 

 ments is appended in a following table which compares this form with 

 C. osceola. 



Affinities. — The only close relative of intrea is the form herein- 

 after described under the name osceola. So far as existing material 

 goes, osceola differs clearly from vitrea in jaw size and details of the sex 

 organs, but the accumulation of specimens from the Gulf Coast 

 region may reveal the presence of intermediate populations. 



