GENERA OF THE SUBORDER ORTHOIDEA 



107 



Superfamily CLITAMBONACEA Schuchert 1929, emended 



Specializing and terminal Orthoidca probably devel- 

 oped out of the Billingsellidx and retaining as a rule 

 the primitive features of deltidia and chilidia, here much 

 enlarged, and developing either pseudospondylia (more 

 rarely) or spondylia simplex. Test impunctate. 



deltidia and cardinalia as in Vellamo and Clitambomtes, 

 but without spondylia and with discrete chilidial plates 

 that may be remnants of chilidia. 



This small family of two genera is restricted to the 

 Lower Ordovician (upper Canadian) of the United 



Tables 



Of European origin and later dispersion 

 to North America 



Clitambonitin^ 



Vellamo 



DELTATRETIDiE 



Polytoechia 



Deltatreta 



Pomatotrema 



» Estlandia 



Hemipronites 



BiLLINGSELLIDiE 



Ingria 



Range, throughout the Ordovician and rarely in the 

 Silurian. 



The phylogeny of this small superfamily is believed 

 to be as shown in Table 8. 



Family DELTATRETID^E Schuchert and 

 Cooper 1931 



Primitive Clitambonacea that probably originated in 

 the Billingsellid.-c. Largely orthoid in expression, with 



States and Canada. It forms a morphologic connec- 

 tion between the orthids on the one hand and the large 

 family Clitambonitida on the other. It includes the 

 two genera Deltatreta Ulrich and Pomatotrema Ulrich 

 and Cooper. 



Internally the dorsal valves are like those of Vellamo 

 and Clitambonites, but there are chilidial plates instead 

 of a chilidium. In the ventral valve there is no spon- 

 dylium, but in old shells of both genera there may be 

 a pseudospondylium. The deltidium of both genera is 

 exactly like that of the Clitambonitidae. 



