Ordovician Inarticulate Brachiopods from Poland and Estonia

New detailed studies make revealed items of taxonomic importance about inarticulate brachiopods of five Ordovician stages: Tremadocian, Arenigian, LIanvirnian, Llandeilian and Caradocian. These studies are based on material etched from the rock of exposures in Poland (Holy Cross Mountains), in Estonia, of the cores of deep borings in north- eastern Poland, from erratic boulders of the Baltic coast and environs of Warszawa. The numerous and best preserved collections come from cores. In general, two group s of the studied inarticulates: acrotretids and siphonotretids are of special interest because of their great diversity. Presumably, they can be very useful in extended correlation. Their range in time and relative stratigraphic values are given. The described species belong to 26 genera and 9 families of the orders Lingulida and Acrotretida. Fourteen new species, 3 new genera and I new subfamily are proposed, based on the features of external morphology and internal structure. Ditreta n. gen., Paratreta n. gen. and Semitreta n. gen. are the Lowermost Ordovician acrotretids recorded in the studied collection. Myotretinae n. subfam. is erected to include forms with a distinctive laterally folded median dorsal septum, Diagnoses of many genera, subfamilies and families are emended by use of some new data obtained in the scanning electron microscope. Elements of external morphology and internal structure are analysed as import ant for taxonomy, some of them (such as structure of a median septum, elements of articulating mechanism, pattern of the surface ornamentation, protegular pitting) being judged as valuable for generic and familial classification. Detailed studies of the inarticulate brachiopods have shed light on the morphogeny of certain formerly obscure characters, e.g. surface concentric ornamentation (much more complicated than one could suppose), shell surface and septal spinosity, median septum, details of the hinge, shell shape and outline - these all being, in addition, considered from the point of views of their probable function and, in some cases, their evolutionary significance. Regarding the problem of growth processes in some acrotretids it appears that, in general, two quite distinct phases for the postprotegulurn stage can be distinguished on the basis of the shell shape and outline and concentration of the surface lines. Shell abnormalities have not been previously discussed in inarticulate brachiopods. These are not numerous and seem to be caused by, among others, desease or the activity of predators.