ALATE SPECIMEN OF ATRYPA RETICl/LARIS 173 



A HIGHLY ALATE SPECIMEN OF ATRYPA 

 RETICULARIS (LINN.) 



A. O. THOMAS. 



The most abundant fossil in the upper part of the Wapsipini- 

 eon beds at Independence and elsewhere in east central Iowa is 

 a fine-ribbed representative of Atrypa reticularis (Linn.). 1 This 

 species is found in every fossiliferous horizon in the Devonian 

 or! the state. Indeed, it is world wide in its distribution and is 

 the "longest lived of all known organisms,'"- ranging from early 

 Silurian through the Devonian into the early Mississippian. 3 

 Generally speaking, however, the species came to an abrupt 

 end with Devonian time although the genus continued on for 

 a brief period into the Mississippian. 4 In a species ranging so 

 widely both vertically and geographically many varietal form- 

 are usually developed. In the Devonian of Iowa nearly every 

 horizon that may be set off at all sharply by lithological or 

 faunal differences has its peculiar variety or imitation of A. 

 reticularis which in some cases perhaps could be well d 

 nated as good species. Such a variety is the fine-ribbed, rather 

 robust form from Independence which has "a tendency to be- 

 come alate at the cardino-lateral angles, and having a form 

 that is decidedly lenticular, particularly in the young and half 

 grown individuals.""' In rare cases the curious marginal alations 

 or fringes are preserved. 



Specimens illustrating marginal alations were obtained by the 

 late Professor Calvin from a quarry in the suburbs of Inde- 

 pendence many years ago. The quarry which furnished the 

 best specimens has long since fallen into disuse so that good 

 examples are now obtained with difficulty. 



The alations or winglike expansions are made up of a num- 

 •ber of thin lamellae which extend from the surface of the valves. 



ilowa Geol. Surv.. vol. viii. 1898. p. 229. 



2 Clarke and Swartz. Maryland Geol. Surv.. Upper Dev., 1913, p. 



3 Herrick: Bull. Sci. Lab. Denison Univ.. vol. iii. 1SST, p. 98, pi. iii, fig. 11; 

 vol. iv, 1888. pi. ix. fig. 7. 



4 For "example. Atrypa infrequens Weller. 111. Geol. Surv.. Monog. T. 191 !. 

 p. 285, pi, xxxv, figs. 1-5, Glen Park limestone (Kinderbook), Glen Park, 

 Missouri. 



6 Calvin; Amer. Geol., vol. 8, 1891, p. 143. 



