ABSTRACT
Empty shells of the extant species Dallina septigera, Hemithyris psittacea, Gryphus vitreus, Laqueus californianus, Liothyrella uva, Neothyris lenticularis, Notosaria nigricans, Terebratalia transversa, Terebratella sanguinea, and Terebratulina retusa were placed alternately on fine gravel (2.0 to 4.0mm) and fine sand (0.20mm) in a recirculating flume in each of nine potential life orientations, including three with the commissural plane erect, three reclining on the dorsal valve, and three reclining on the ventral valve. Mean threshold current velocities to transport each species were determined in each orientation on each substrate. Velocities for shells resting on gravel ranged from 15 to 40cm/sec. Velocities for shells in the same orientation resting on sand usually averaged 5 to 10cm/sec lower. The most stable reclining orientation for all species on either substrate was with the posterior margin upcurrent. Velocities for erect shells averaged 1 to 4cm/sec higher when a lateral margin faced upcurrent for eight of ten species. Posteriorly weighted shells (T. transversa) were more stable relative to elongate (e. g., L. uva), globose (e. g., L. californianus), or conical (e. g.,H. psittacea, T. sanguinea) shells. Eddygenerating plicae (D. septigera) and costae (T. sanguinea; N. nigricans) decreased shell stability relative to similar sized costellate (T. retusa; H. psittacea) or smooth (G. vitreus) shells.
