174 IOWA ACADEMY OF SCIENCE 



;: rising from what are generally regarded as lines of growth on 

 the ordinary Atrypa shell. These concentric lines, however, are 

 rather more than records of halts in the growth of the shell, in 

 appearance they approach varices where the plications are slight- 

 ly dilated and elevated as may be seen on shells from which the 

 lamella? are removed. Each lamella extends outward in such 

 a way as to make a small angle with that part of the shell proper 

 which continues beyond the line of their common union. The 

 successive lamellae lie more or less closely one upon another near 

 their bases but out toward their margins they are considerably 

 separated and the spaces between them are filled with the ordi- 

 nary matrix in which the shells are preserved. There is no 

 evidence that the lamellae ever coalesced. Their surfaces partake 

 of the characteristic markings of the shell itself and the plica- 

 tions or ribs on the lamellar surfaces are continuations of those 

 en the shell; with growth the plications increase in size, bifur- 

 cate, and so on, as do those which continue over the shell. The 

 lamellar surface is wrinkled and uneven in contrast with the 

 smooth evenly rounded surface of the valves. As seen in sec- 

 tion the lamella? vary in thickness and the outer and inner 

 surfaces of each lamella are similarly plicated, that is, each 

 lamella is a rigid corrugated layer. The plications on one lamella 

 do not coincide either in size or always in direction of growth 

 with those on the surfaces of adjacent lamella? immediately 

 above or below. 



The alation is developed in a plane roughly parallel to a plane 

 passed between the valves; its lateral development along the 

 posterior margin gives the shell the appearance of having a long 

 straight hinge-line; anteriorly the lamella? bend to conform to 

 the sinuosity of the front margins of the valves. 



The hardness of the rock in which the Iowa specimens occur 

 and 1 lie fragility of the lamella? make it difficult to disengage 

 a complete specimen. The one here figured is so broken along 

 the margin that the full size is not known. Even fragmentary 

 preservation is rare; the shells showing alations in the collection 

 ,at hand as well as those seen in the field are usually mature 

 and old individuals, — more frequently the latter, since "those 

 [lamella?] upon the umbonal and median surfaces of the valves, 

 have been worn off during the life, or before the fossilization 

 of the shell." 



< ; Hall and Clarke; Pal.. N. V. vol. viii. pt. ii. p. 168, Albany. 1894. 



