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By Kennedy W.J., Cobban W.A.

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Disappearance of a hyponomic sinus and development of ventral rostra have also been interpreted as indicating the loss of a funnel necessary for jet propulsion (Mutvei and Reyment 1973). Donovan (1964) claimed that all post-Permian ammonoids lack a hyponomic sinus, it being replaced by a projecting rostrum and the change occurring somewhere near the point of origin of the Xenodiscidae. He concluded that 'in view of the presence of the sinus in all other shelled cephalopods, its loss is likely to have been a significant evolutionary change, and may have accompanied the disuse of the funnel as a locomotory organ' (1964, p.

1, figs. 1-5) has led to a number of taxonomic problems, for dimorphs have usually been referred to different species, and frequently to different genera or even E X P L A N A T I O N Intraspecific OF PLATE 9 variation F i g s , la-c, 2a-c. Tropites subquadratus Silberling. U S N M 128262 a n d 128265. C o m p r e s s e d a n d depressed e n d m e m b e r s o f the variation series f r o m the late Triassic L u n i n g F o r m a t i o n o f the S h o s h o n e M o u n t a i n s , Nevada. F i g s .

In Zululand the group reappears in large numbers, again with high variability, under the name of A. flexuosum (of which A. crassiornatum Crick, A. expansum Crick, A. robustum Crick, A. quadratum Crick, A. hippocastanum Crick (non Sower by), and A. latum Crick are probably synonyms). In Texas a rather similar group of ammonites is represented by the AMMONITE ECOLOGY 41 A. stephensoni Adkins, A. adkinsi Stephenson-v4. , tarrantense Stephenson complex. Individuals from any one population would pass unnoticed in any other, but over-all population structures are clearly different.

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