164 U.S. NATIONAL MUSEUM BULLETIN 259 



isolation of founder populations in southwestern San Saba Co., 

 Tex. At that time I regarded this species as being closely related 

 to northern populations of S. balconis, but since then I have relegated 

 some of these northern populations to S. bifurcatus (see Systematics), 

 and the reference to morphological alliance mentioned in that paper 

 should apply specifically to S. bifurcatus and not to S. balconis. 

 Furthermore, the single male from Neel Cave in Menard County, 

 which was referred to S. reddelli by me previously (Holsinger, 1966), 

 is no longer regarded as conspecific as a result of the present study. 

 As presently understood, S. reddelli is restricted to Whiteface Cave 

 in San Saba County. 



According to Reddell (in litt.), the limestone in which Whiteface 

 Cave is developed is virtually isolated from the cavernous limestone 

 of the eastern part of San Saba County (viz., location of Harrells 

 and Germans Caves) by a noncavernous stratum, which has resulted 

 from block faulting. This cave is further isolated from caves to the 

 south, within the Edwards Plateau per se, by noncavernous Pre- 

 cambrian rocks, which are brought to the surface by the Llano uplift. 

 To what extent Whiteface Cave is isolated from caves on its western 

 side is unknown except that about 35 mUes west of this cave there 

 is a major stratigraphic change from Ordovician to Cretaceous bed- 

 rock. Both morphologically and distributionally, S. reddelli appears 

 to be a product of extreme isolation of precursor populations to a 

 restricted area of subterranean drainage in the vicinity of its present 

 range. 



S. russelli is the most highly variable species in the genus and 

 occupies a rather extensive range throughout most of the cavernous 

 area of the eastern Edwards Plateau region. The broken, circular 

 distributional pattern of this species is shown in figure 33; six gaps 

 of 50 to 60 mUes each occur between single cave populations or 

 clusters of populations. Attempts to specifically relate morpho- 

 logical variation to definite geographic patterns have not been en- 

 tirely successful, but several incipient trends have been noted. Quan- 

 titative variation in the ratio of the length of the first antenna to the 

 length of the body in individuals of seven populations of this species 

 is shown in table 5 (see p. 100). Other points of major and minor 

 variation have also been discussed and need not be repeated at this 

 point. 



Basically, patterns of variation, while irregular, tend to indicate 

 that individuals in populations from caves just north and east of the 

 Llano uplift area (viz., Gormans, Harrells, Tippits, and Nolan Creek 

 Caves) are morphologically homogeneous and appear to share in a 

 common gene pool. Four caves located just northwest of Austin 

 (Travis County) contain populations that are morphologically similar 



