Dictyonella reticulata brachiopod from the Waldron Shale

Specimens of Dictyonella reticulata are easy to pick out of a lineup. They have a very distinctive exterior that looks like hatch marks or it is covered with a bryozoan. In actuality the pattern is related to a defensive mechanism whereby the each diamond shaped pit had a small tube extending into the shell. Through that tube the brachiopod could ooze a noxious chemical to keep predators from eating it and opportunistic larvae from settling on it. Think of them as the prototype to having actual spines.

Pedicle valve

Anterior

Brachial valve

Posterior

Profile

The above specimen came from the Waldron shale in Indiana which is Silurian (Sheinwoodian to Homerian stage) in age.

There are some who would ID this fossil as an Eichwaldia reticulata which is an older name.
A review of the "Treatise on Invertebrate Paleontology - Part H - Brachiopoda V1" edited by Moore (1965) has the following description of the two genera (which are in the same family) on page 360:

Eichwaldia BILLINGS, 1858, p. 190 [*E. subtrigonalis; OD].
External ornament only of fine concentric growth lines. Interior of brachial valve
with prominent median septum extending nearly to anterior margin, small boss at posterior end of septum possibly functioning as cardinal process;
musculature unknown. M.Ord., Can. FIG. 229,

Dictyonella HALL, 1868, p. 274 [* Atrypa coralifera HALL, 1852, p. 281; OD] [=Dyctionella OEHLERT, 1887, p. 1267 (nom. null.)].
Very similar to Eichwaldia, differing in its ornament of rather coarse pits defined by intersecting, narrow, elevated lines. Sil., N.Am.~Eu.·Asia. FIG. 229,2.

Volume 8, part 2 of "the Paleontology of New York" by James Hall describes Eichwaldia on page 307 and illustrates it on plate LXXXIII (88 for those unfamiliar with roman numerals).

Also, Mike over at Louisville Fossils points out that the genera Dictyonella was renamed Eodictyonella in 1994 by Anthony D. Wright in his paper titled "Eodictyonella, a new name for Dictyonella Hall, 1868, not Dictyonella Schmidt, 1868" in the Journal of Paleontology, July 1994, v. 68, p. 908-909

Here is a Dictyonella corallifera from the Irondequoit limestone of New York which is roughly the same age as the Waldron shale.