ATOPETHOLID MILLIPEDS — HOFFMAN AND ORCUTT 105 



serves the function of providing rigidity to the entire structure. Species 

 of the Atopethohnae have increased the size and efficiency of the 

 vinculum, which takes up much of the intercoxal space, and is pro- 

 longed caudad between the coxites, terminating in a ligament that 

 extends laterad on each side to insert on the caudad side of the sternal 

 apodeme (fig. 2,c, No. 5). The greatest development of the sternite 

 is found in the subfamil}^ Onychelinae, where it is prolonged distad 

 into a triangular process very similar to the sternite of species of the 

 Rhinocricidae (see fig. 8, a) . Here there is no spacer between the coxites, 

 rigidity of form apparently being accomplished by a certain amount 

 of fusion of coxites to the sternite. 



The coxites are somewhat variable in form throughout the group, 

 but are similar to the typical spiroboloid form in being produced into 

 a slight apex on the mesial surface, this character reaching its greatest 

 extreme in Arinolus (fig. 11, a), which is apparently the most specialized 

 genus in the family. In all genera, the coxites are produced proximad 

 into slender, acute coxal apodemes that serve for muscle attachment. 

 As seen in caudal aspect (figs. 2, a; 4,6; 8,6), the mesial edge of the 

 coxite is drawn out gradually to form the apodeme, which may be 

 either flat and simple (fig. 8,6) or rofied to form a concavity (fig. 4,6) 

 with the free caudal edge joining the coxite near its caudomesial end. 

 In all cases, the coxite forms a gonocoel cavity in which the posterior 

 gonopod is carried. 



The telopodite joint is normally rather small and unmodified. 

 Usually it is produced into a blunt, laterall}^ du'ected tip, with a 

 departure from this plan occurring only in Atopetholus. Species of 

 that genus are characterized by an additional accessory process that 

 has been referred to in the literature as "posterior apophysis," 

 "digitiform process," and "posterior finger" by Chamberlin and that 

 appears to be possibly homologous to similar processes described by 

 Verhoeff (1924) for some Australian spirobolellid genera. There is a 

 membranous, flexible articulation between the coxite and telopodite, 

 but within the gonopod no major muscles have been detected that 

 might activate the distal joint. Apparently there is an evolutionary 

 tendency for the size of the telopodite to increase, it being smallest in 

 Comanchelus and largest in Arinolus, genera that are phylogenetically 

 opposed in numerous other characters as well. 



The posterior gonopods lie concealed within the anterior pair, their 

 coxites exposed and directed toward the median line, often with their 

 ends in contact. There is, however, no trace of a sternal connection, 

 and it will be recalled that in the Trigoniulidae, where such a sternite 

 persists, the coxites are directed caudad in line mth the major body 

 axis. There is a large apodeme, inferentially homologous with the 

 sternal apodeme of the anterior gonopods, attached near the mesial 



