NO. 2037. NORTH AMERICAN SPRING-TAILS— FOLSOM. 461 



Description. — Ungues tridentate or quadridentate (pi. 40, fig. 3). 

 Ungiiiculus unidentate. Dental spines nine to eleven, rarely twelve, 

 the two proximal and the two distal considerably larger than the rest 

 (pi. 40, fig. 4). Dental formula typically 2/5-7, 2; rarely 2/8, 2. 

 Length 4 mm. 



Variation. — The body color, clear yellow in Alaskan specimens, is 

 highly variable in specimens from other parts of the United States. 

 Thus it may be cream yellow, straw yellow, bright ferruginous or 

 grayish; the grayish specimens frequently having elongate rounded 

 oblique whitish spots on the sides of the thorax and abdomen. In 

 Illinois I have collected at one time specimens illustrating all these 

 color variations from a single square foot of soil. A variable amount 

 of dark pigment occurs on the anterior border of the mesonotum, on 

 the coxse, tibiae, and distal ends of the femora, and on the abdominal 

 segments and the manubrium. 



The teeth of the ungues are commonly 4, 4, 4; 3, 3, 3; or 4, 3, 3. 

 The tooth of the unguiculus is occasionally absent. The intermediate 

 teeth of the mucrones vary from six to ten in specimens three to four 

 millimeters in length, with an average of eight or nine. 



Tomocerns americanus can not be regarded as a distinct species, 

 since it intergrades with typical Jlavescens in all the characters that 

 are of specific importance, as I showed in my paper on Alaskan 

 Apterygota ('02, p. 99). Such intergradations as these occur: 



Teeth of unguis. Dental formula. Locality. 



2, 2, 2 2/4, 2 Alaska, Tennessee, 



2, 2, 2 2/5, 2 Alaska, Illinois. 



4, 3, 3 1/5, 2 Indiana, ^lissouri. 



Frequently the same individual has spines oi jlavescens on one dens 

 and those of americanus on the other. 



One of my specimens from Indiana is especially interesting as a 

 transitional form. The spines of the right dens are those of typical 

 jlavescens. Those of the left dens ave jlavescens varying into ameri- 

 canus; for the second large basal spine is represented by a greatly 

 thickened hair; while on the right dens in the corresponding place 

 there is a simple hair. 



Synonymy. — Of the specimens referred to T. plumheus Linnaeus by 

 Packard ('73, p. 38), those from Texas are americanus. The speci- 

 mens, three in number, labeled ''Waco, Tex., Belfrage," were studied 

 by me in the Museum of Comparative Zoology, Cambridge, Massa- 

 chusetts, m 1897-98. They had quadridentate ungues. Only one 

 specimen, however, retained enough of the furcula to show the spines; 

 these were 2/5, 1 on the right side and 2/3, 2 on the left. 



The form regarded as T. arcticus by Guthrie ('03, p. 79) is ameri- 

 canus, as is evident from his description and figures as well as from 

 seventeen of his original specimens that I have examined. 



