| Based on optical and SEM microscopic observations, the projections of the outer surface of the periostracum and inner micro-structures of the shell are described and redefined. The outer surface of the periostracum is practically smooth in both species. Considering a mesoscopic view of the periostracum, A. trapesialis (Lamarck, 1819) presents regular corrugations in the form of radial sequences of arches on the disk region, isolated rays or horizontal sequences of rays on the anterior lower region. A. elongatus presents corrugations formed by series of oblique arches on the disc and oblique rays on the carina. Under SEM, micro ridges were more evident in A. elongatus, but a wide diversity of shapes and patterns of micro fringes were observed in A.... |
