ALATE SPECIMEN OF ATRYPA RETICULARIS ITT, 



This feature on A. reticularis was pointed oul and illustrated 

 sixty years ago by the Sandberger brothers on a specimen Prom 

 the Rhenish Devonian of Germany. 7 Davidson" described and 

 illustrated some interesting examples of A. reticularis with 

 "foliated expansions" from the Wenlock limestone, Silurian, of 

 England. Whiteaves 9 figured a specimen from the Devonian of 

 Canada. His figure illustrates the lamellae remarkably well. 

 Its greatest width is 14 millimeters more than that of the speci- 

 men here illustrated from Independence. Clarke and Swartz 10 

 discuss this feature on specimens of this species from the Devo- 

 nian of Maryland. Other references could easily be added bul 

 these will suffice to show that this feature is not limited to the 

 Atrypas of any given locality. Moreover, it seems to have been 

 a characteristic of A. reticularis at various times throughout 

 its history and doubtless was developed to a greater or less ex- 

 tent on several of its many varieties. What the purpose of 

 these excrescences could have been we can only -conjecture. Such 

 seemingly useless and extravagant skeletal matter in many cases 

 presages racial old age and final extinction but their presence 

 on members of the species in the Silurian soon after the species 

 had made its appearance seems to preclude this explanation. It 

 is quite possible that short lived offshoots of the species, destined 

 to disappear, developed these encumbrances during their later 

 stages. 



The alate specimen which is the subject of this article has a 

 maximum width of 10 cm. and a length of 6.5 cm.; the "hinge- 

 line" is 7.3 cm. long. The lamellae which are preserved are all 

 outgrowths of the pedicle (ventral) valve, those formerly on the 

 brachial (dorsal) valve having been almost wholly broken away ; 

 the width of the alation on the specimen averages three centi- 

 meters. 



Specimens from the same bed, on which the alations are nol 

 preserved, show the usual expression of the species. The non- 

 lamellate specimens illustrated in the accompanying plate are 

 quite similar to those described and illustrated from the same 

 bed by James Hall in 1858. 11 



7 Die Verstein. cl. Rliein. Schicht. in Nassau, pi. x.wiii. fig. l, Weisb; 

 1856. 



s British Sil. Brach., pp. 129-133. pi. xiv, figs. I. 2. I Ion, L867. 



°Contr. Can. Pal., vol. I, pt. iv, p. 289, pi. xx.wii. fig. 8; Ottawa. 1892. 



"Maryland Geol. Surv., Upper Dev., p. 586, pi. Iv, figs. 6, 10; Baltimore, 

 1913. 



"Hall's Geol. of Iowa, vol. I, pt. ii. p. 515, pi. vi, figs. I. 5. 



