Larval development, musculature, and relationships of Sinuitopsis and related Baltic bellerophonts

Sinuitopsis neglecta Perner (2 = Pharetrolites elegans (Kroken)), a bellerophont occurring in the Ordovician of the Baltic region and Bohemia, has bilaterally symmetrical 'metameric' muscle scars similar to those of Cyrtolites. Spirally coiled monoplacophorans with Cyrtolites-like muscle scars are shown to
be closely related to the bellerophontid 'gastropods' with unknown muscle scars. This is indicated by the time gradients in both the characteristics of conch morphology and mode of larval development. Volkhovian Sinuitopsis(?) sp. has weakly separated small embryonic shell; Volkhovian to Uhakuan Modestospira sp. shows a distinct boundary between smooth small embryonic and strongly ornamented remaining (larval ?) shell; involute Lasnamiigian Kokenospira estona (Koken) has relatively large, smooth embryonic shell with a sharp boundary between embryonic and adult ornamentation. The synonymizing of Cyrtonellida Horny, 1963 with Bellerophontida Ulrich & Scofield, 1897 and their
inclusion into Monoplacophora is proposed.