11
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March
132
140
ROLLINS, H. B., BATTEN, R. L. 1968. A sinus-bearing monoplacophoran and its role in the classification of primitive molluscs. Palaeontology, 11, 1, 132–140.
H. B. Rollins and R. L. Batten The discovery of symmetrically disposed multiple muscle scars in Bellerophon acutilira Hall, a middle Devonian form assigned to the bellerophontoid genus Sinuitopsis Perner, indicates that Sinuitopsis is a cyclo-myan monoplacophoran. Deployment of the muscle scars shows affinity with the cyclomyan genera Yochelso-nellis (Horny) and Cyrtolites Conrad. A ridge-furrow complex is interpreted as a reflection on the inside of the shell of a dorsally situated rectal tract. The presence of a deep anal sinus and notch in S. acutilira demands a reconsideration of monoplacophoran-gastropod relationships. Such a sinus or slit can no longer be always considered anterior, and thus, evidence of torsion in fossil archeogastropods. Quite possibly sinus-bearing cyclomyans were the direct ancestors of the primitive archeogastropods (i.e. the Bellerophontina). The use of the sinus as a tool for the recognition of bellerophontids is discouraged in favour of parietal deposits and posterior trains. A posterior anal sinus was probably advantageous in the achievement of maximum separation of respiratory currents and excretory products. The Palaeontological Association (Free Access)