PART V. THE GENERA OF THE SUBORDER PENTAMEROIDEA 



Derived out of the Orthacea, the Pentameroidea 

 retain their impunctate tests, but most of them tend 

 with time to lose more and more of their transverse 

 shells with wide interareas and to become elongate and 

 subrostrate, smooth or costate, with well developed 

 spondylia and characteristic cardinalia. The delthy- 



rium is usually open but may be modified by deltidia 

 or marginal thickenings. 



The Pentameroidea begin in the Middle Cambrian 

 and die out with the Devonian. They include the 

 superfamilies Syntrophiacea Schuchert and Cooper and 

 Pentameracea Schuchert. 



Superfamily SYNTROPHIACEA Schuchert and Cooper 1931 



Specializing Protremata derived out of the Orthacea 

 (probably the ancestral stock of the Billingsellidas ) , 

 developing a more or less lobate exterior, interiors with 

 either pseudospondylia or spondylia of the simplex type, 

 and occasionally cruralia. Cardinal process absent or 

 rudimentary. Delthyrium and notothyrium open. 

 Test fibrous, impunctate. 



Geologic range. — Middle and Upper Cambrian, 

 Ozarkian, and Lower Ordovician. 



Embraces the Clarkellidae, Syntrophiidx, and Huen- 

 ellid<e. 



The Syntrophiacea may have given rise to the Pen- 

 tameracea. It is probable that the other stocks with 

 simple spondylia — the Clitambonacea — arose in the 

 Billingsellidae, retaining and developing more fully 

 the shell form, chilidium, and perforate deltidium; in 

 other words, remaining more like Orthacea than 

 Pentameracea. 



The genetic relations of the Syntrophiacea are 

 thought to be as shown in Table 16. 



Table 16 



Clarkellid^ 



HUENELLIDiE 



Clarkella 



Huenellina 



Yangtzeella^ 



Syntrophina 



-> Huenella 



SVNTROPHIIDiE 



Syntrophioides 



^ Syntrophia 



Orthacea 



(probably out of an early Middle Cam- 

 brian ancestor of the Billingsellidae) 



