80 BULLETIN 168, UNITED STATES NATIONAL MUSEUM 



subovate, acute or acuminate, with or without a tooth at the angle 

 of the inner lamella. Hind claws the largest. Tenent hairs 2, 3, 3, 

 strongly knobbed, distally bending. Furcula appended to the fifth 

 abdominal segment, long, extending almost or quite to the ventral 

 tube. Manubrium relatively short and stout, udth many setae 

 dorsally and ventrally. Dentes two and one-half to three times as 

 long as the manubrium, not convergent, slender, gradually tapering, 

 dorsally crenulate, with a strong ventral subapical seta that may 

 extend as far as the apex of the mucro (pi. 27, fig. 298), or beyond it. 

 Mucro (pi. 27, figs. 298, 299) shorter than hind unguiculus, triden- 

 tate, with the teeth somewhat variable in relative sizes: Apical tooth 

 the longest, slender or stout, more or less hooked; second inclined 

 caudally; third lateral, close to second, conical or slightly hooked, 

 inclined caudally. Rami of tenaculum quadridentate; corpus with 

 several (7 to 12) ventral setae, the largest of which (on the last three 

 abdominal segments) are minutely and unilaterally branched; erect 

 simple sensory setae are present also. Maximimi length, 2 mm. 



Remarks. — In occasional individuals the genital and anal segments 

 are not completely ankylosed. 



European specimens may attain a length of 2 mm (Agren); but 

 the largest individuals that I have seen from this country (from 

 Minnesota) were 1.2 mm in length. 



Specimens that I collected in Massachusetts agree exactly with 

 German examples received from Dr. C. Schaffer and with English 

 specimens from J. M. Brown. 



The types of MacGillivray's parva and determinata in the Museum 

 of Comparative Zoology, Cambridge, Mass., are sensibilis Tullberg. 

 In the original description of parva the third, or lateral, tooth of the 

 mucro is not mentioned as being present. 



Guthrie's specimens from Minnesota are evidently sensibilis. 



In Europe, I. sensibilis lives primarily under loose bark and in 

 moss, but occurs also under logs and stones, on the seashore under 

 seaweed, and on the surface of pools of fresh water. It occurs at 

 times in large masses, and has been taken abundantly on snow on 

 mild ^vinter days. 



Most of my North American material of this species was taken 

 under the loose dead bark of trees or logs: Chestnut, cedar, oak, bass- 

 wood, maple, willow, and black locust. 



Distribution. — Recorded as follows: 

 Massachusetts: Cambridge, May 7. Dedham, July 21. Dover, November 9, 

 A. P. Morse (Museum of Comparative Zoology). Newton, June 22. Nor- 

 wood, August 26. Waltham, July 29. 

 New York: Macedon, May 11, J. D. Hood. 



Ohio: Salineville, A. D. MacGillivray (Museum of Comparative Zoolog)')- 

 Illinois: Oregon, April 3, in moss on rocks, T. H. Frison and H. H. Ross (Illinois 

 State Natural History Survey). 



