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Petrushevskaya 1967

Fig. 34
Phorticium clevei (J├Ârgensen):
Successive stages of growth of skeleton: I - specimen in which only the central part of the skeleton is formed, view from above; II - anterior view; III - growth of following turn, lateral view; IV - following stage in same position as in II; V - optical section through shell approximately at same position as in III.