220 PROCEEDINGS OF THE ACADEMY OF [Feb., 



Dakota. All the new species described in the present paper belong 

 to the group with 12-jointed antennae, and it is probable that many 

 more members of this group remain to be discovered in the West and 

 Southwest. Both groups are represented in ^lexico and Central Amer- 

 ica. To judge from Emery's table of the South American species, 

 those with U-jointed antennae predominate again south of the Equator. 

 Most of these species, however, whether having 11- or 12-jointed 

 antennae, have acute, projecting angles to the pronotum, and are there- 

 fore consigned to a particular subgenus, Goniothorax, by Emery.* 

 The small group comprising the subgenus Dichothorax Emery (possibly 

 monotypic) is confined to the Southern United States. This subgenus 

 resembles the subgenus Temnothorax j\Iayr (including only T. recedens 

 Nyl.) in many respects. It is interesting to note that this form occurs 

 only in Southern Europe. 



While some of the European Leptothorax (like tuberum and uni- 

 fasciatus) are known to be extremely variable, the North American 

 materials at the disposal of previous writers have not been sufficient 

 to prove the same for any of the species on this side of the Atlantic. 

 Nor am I able to throw as much light as I could wish on the limits 

 of variabihty in our species, although my material certainly shows 

 that some of our species are decidedly unstable. Such are, e.g., L. acer- 

 vorum, curvispinosus , nitens and possibly also Schaumi and fortinodis, 

 especially if the two latter realty represent extreme forms of the same 

 species, as seems to be indicated by the existence of intermediate 

 forms. 



The genus Leptothorax was established by ^layr in 1855,' on a num- 

 ber of species previously included by Nylander and other myrmecolo- 

 gists in the composite genus Myrmica, a genus which at one time 

 contained practically all the known ants of the subfamily Myrmicinae. 

 Though some of the characters of Leptothorax are not very definite, 

 the genus has nevertheless stood the test of nearly half a century and 

 will probably continue to stand. Like many ant-genera, and genera 

 of other animals also, for that matter, it is recognized not so much by 

 a description of its characters, as by its peculiar and almost unmistak- 

 able habitus. He who has had little experience in handling ants will 

 be liable to confound the workers of Leptothorax with the workers of 

 Pheidole or vice versa, but to the experienced eye even the gait of the 



*'Studi sulle Formiche della Fauna Neotropica," Bull. Soc. Ent.Ital. Ann., 

 XXVIII, 1896, pp. 26, 27. 



'• ■' Formicina Austriaca," Yerh. K. K. zool.-hot. Ver. Wien, Bd. 5, lSo.5, pp 

 431-433. 



