46 Aug. F. Focrstc 



is more closely related to this species than is Lcptaciia richmondoi' 

 sis, which frequently is identified with the Caradoc species. 



The Saltillo limestone is identical with the Hermitage lime- 

 stone of the area covered by the Columbia folio, in Tennessee, and 

 with the Logana of Kentucky. 



The Lcptacna from the Saltillo limestone is interesting chiefly 

 because it indicates one of the earliest appearances of this genus 

 in strata whose equivalent can be recognized in Kentucky. Lcp- 

 tacno cJiarlottac, of Minnesota, may be an earlier form. 



The succession of forms, in descending order, is as follows : 



Lcptacna ricJiiiioiidoisis Waynesville to Whitewater. 



Lcptacna richnwndcnsis-prccursor . . ..Arnheim. 



Lcptacna gibbosa, James Economy. 



Lcptacna invcnusta Fulton or Lower Economy. 



Lcptacna tcnuistriafa, var Saltillo (Logana). 



Plectorthis equivalvis, Hall. 



(Plate II. figs, is A.B: Plate VI, tigs. 17 A.B, 2.3.) 



Plectorthis equivalvis was described by Hall from the Cincin- 

 natian series at Cincinnati, Ohio, and a specimen from the Trenton 

 at Middleville. New York, was doubtfully referred to the same 

 species. The type specimen, from which figures 6 a to c. on plate 

 32, of the Nciv York Paleontology was prepared, is not listed 

 among the types preserved in the American Museum of Natural 

 History in New York City, but a specimen bearing one of the 

 green tags used by Hall to indicate his types occurs among a group, 

 numbered 4490 and labelled Plectorthis fissicosta, and appears to 

 ha^■e been the specimen used in the original description of Plector- 

 this equivalvis. 



The length of this specimen is 15 mm., the width across the 

 middle is 16 mm., the length of the hinge-line is 13 mm., and the 

 thickness or depth of the shell is 8.3 mm. The contraction in 

 front of the hinge-line is considerably less than is indicated in the 

 drawings accompanying the original description, but similar errors 

 occur in other figures on the same plate. The foramen is not 

 exposed. The number of primary plications is about 20, increas- 

 ing soon to 23, and about 6 millimeters from the anterior margin 

 of the shell these primary plications are sup]:)lemented on each side 

 by a secondary plication, thus forming fascicles of plications, con- 

 sisting in each case of three plications, the primary plication being 

 more conspicuous than the other two. The surface of the shell 



