Pterocystis pyriformis (Dürrschmidt, 1987) Siemensma, 1991
Diagnosis: Periplast 8-15 µm in diameter (measurement based on whole mount); details of protoplast unknown. Scales of two types. Spine scales almost pear-shaped, 1.5-2.0,0 µm long. The shaft 1.4 µm long, 0.06 µm thick, is hollow, distal half without wings and tapering to a blunt apex. The lateral wings and the basal wing join to the peak-like appearance. The wings are smooth, their outer edge is slightly curved inwards. The area around the shaft base is depressed concavely, giving the impression of a scoop. Plate scales, elongated elliptical or ovoid, 1.4-1.6 µm with a narrow peripheral rim and a central swelling, sometimes developed only as an axial thickening.
Remarks: P. pyriformis most closely resembles P. paliformis but differs in the form and size of the spine scales and in the appearance of the plate scales. Also in P. pyriformis the plate scales reveal a central swelling, which demonstrates the close relationship of this species to P. paliformis and E. erinaceoides. The gradual reduction of the central swelling can be traced back from E. erinaceoides over P. pinnata, P. paliformis to P. pyriformis and finally perhaps to P. foliacea. During this process the shaft becomes more and more reduced. It is, therefore, supposed that both characteristics are controlled by the same gene centre.
Distribution: Chile, New Zealand.