ATRYPA. 115 
1867. ArvrRypa RETICULARIS, Hall. Pal. N. Y., vol. iv, pt. 1, p. 316, pl. li, figs. 
10—24; pl. lii, figs. 1—12; pl. li, figs. 
3—19; pl. liia, figs. 22, 23. 
1879. — — Barrande. Syst. Sil. Bohém., vol. v, pl. xix, 
figs. 1—19; and pl. cix, figs. 2, 1-13, 
Kt. E and F. 
1882. — — Davidson. Brit. Foss. Brach., vol. v, pt. 1, p. 39, 
pl. i, fig. 16. 
1882. _ = Barrois. Mém. Soe. Géol. Nord, vol. ii, p. 265. 
1885. — _ Maurer (pars). Abhandl. Grossh. Hessisch. Geol. 
Landes., vol. i, pt. 2, p. 180, 
pl. vii, figs. 30, 30a, 37—37 c. 
1886. _ - Stuckenburg. Mém. Acad. Imp. Sci. St. Pétersbourg, 
ser. 7, vol. xxxiv, No.1, p.7, pl. iii, 
fig:9) 
1886. _ — (DEsquaMaAtA), Wenjukof Faun. Dev. Syst. N.W. 
und Cent. Russl., p. 514, pl. vi, 
figs. La, b, 5a, b, 6. 
1889. - — Toll. Mém. Acad. Sci. St. Petersbourg, ser. 7, vol. 
xxxvil, No. 3, p. 24, pl. 1i, figs. 14a, b, d, e. 
1890. — — and var. AspERA, Hhlert. Bull. Soc. Géol. Fr., ser. 
3, vol. xvii, pl. xx, figs. 4—65 a. 
1892. — — Whiteaves. Contrib. Canad. Paleont., vol. i, pt. 4, 
p- 289, pl. xxxvil, fig. 8. 
Remarks.—A. recticularis, A. desquamata, A. aspera, A. compressa, are all 
common, and A. trigonel/a is not rare in the English Middle Devonian; but 
partly from their great variability, and partly from the imperfect state of most of 
the specimens, it is most difficult to decide their specific values or limits. I have 
examined very large numbers of specimens; and, while the differences are so 
great that it is almost impossible to imagine them all to be the same species, 
the connecting links are such that it is almost equally impossible to regard them 
as distinct. 
Certain marked distinctions may be traced in shells of all sizes, and, were all 
our specimens perfect, it might prove possible to find clear dividing lines between 
them; but in the condition in which they generally occur it is often almost 
impossible to allocate them. 
Some of the chief forms may be thus defined : 
1. A. reticularis : beak small, narrow, much incurved, and with rounded sides ; 
foramen nearly or entirely hidden; ribs generally fine and numerous, but some- 
times coarse ; shape circular, flat when young, but very globose when aged. 
2. A. desquamata (including A. compressa) : beak large, wide, erect ; area large, 
wide, flat, with angular sides, and showing deltidium, which is sometimes very 
large; foramen well exposed, large, oval, and more or less distant from the apex 
