Shell microstructure of Cornulites semiapertus Öpik, 1930 and other early cornulitids from the Ordovician of North Estonia

The shell microstructure of Ordovician Cornulites species is in general similar to Silurian species. The earliest cornulitid from Baltica, C. semiapertus Öpik, 1930 (Darriwilian), has vesicles, lamellar shell structure, but no pseudopunctae. The earliest Cornulites with pseudopunctae appears in the Katian. The initial shell structure of cornulitids may have been microlamellar, but without pseudopunctae.