Shell Microstructure of Strophomenide Bilobia Cooper (Brachiopoda) from the Late Ordovician of Baltoscandia

The shell microstructure of two species of Bilobia, B. musca (Öpik, 1930) and B. alichovae Madison, 2017 (family Leptestiidae Öpik, 1933) from the Late Ordovician of Baltoscandia was studied. Possible primary layer, fibers, pseudopunctae, extropunctae, and the only known for strophomenides hollow punctae are described. Possible diagenetic changes of different skeletal elements in different parts of the shell are appreciated. The assumption of A. Öpik (1933) on the presence of hollow punctae in B. musca is confirmed.