128 SHELVE DISTRICT 



Although only plectambonitaceans are currently known to have been contem- 

 poraneous with Eocramatia, representatives of the other two strophomenidine super- 

 families, the Strophomenacea and Davidsoniacea, first occur in the Upper Llanvirn 

 and basal Caradoc respectively and are stratigraphically eligible for consideration 

 as related stocks. From a general morphological viewpoint the pseudopunctate 

 strophomenaceans are unlikely to be related, but whether the new genus should be 

 assigned to the plectambonitaceans or the davidsoniaceans mainly depends on the 

 interpretation of the plates filling the notothyrium. These structures are like the 

 chilidial plates of the leptellids and, in conjunction with the planar brachial valve 

 and the absence of a notothyrial platform, suggest that the new genus is a plectam- 

 bonitacean. On the other hand, the posterior surfaces of the plates appear to have 

 been grooved in life and, therefore, may have functioned as supports for the dorsal 

 diductor muscle bases which were, in contrast, attached to the notothyrial floor 

 between the chilidial plates in plectambonitaceans. If this were so the plates are 

 homologous with the bilobed cardinal process and, together with the impunctate 

 shell condition and simple teeth, hint at a davidsoniacean relationship. The 

 principal features refuting this interpretation are the attitude of the socket ridges, 

 the absence of dental plates and the planar brachial valve ; and, since simple teeth 

 without flanking accessory processes and an impunctate shell are known to have 

 persisted among the most primitive plectambonitaceans such as the contemporaneous 

 plectambonitids and taffiids, it seems more appropriate to assign Eocramatia to the 

 Plectambonitacea at least until better preserved material is available to decide the 

 issues in question. 



Having had so much difficulty in assigning Eocramatia to a superfamily it is not 

 surprising to find that its familial connections within that taxon are at present elusive. 

 In some respects Eocramatia is closest to the Taffiidae, but both the impunctate 

 shell and the structure of the cardinalia militate against including it in that family 

 and, in association with other features, call for the erection of a new monotypic 

 family. 



Genus EOCRAMATIA nov. 



Name. An early brachiopod with a mixture (Gk. Kpdfia) of morphological features. 



Diagnosis. Subquadrate, slightly uniplicate plano-convex shells widest at the 

 hinge-line, with a shallow median sulcus in the pedicle valve corresponding to a low 

 median fold in the obscurely sulcate brachial valve ; ornamentation costellate by 

 branching and intercalation tending to thicken between intersections with concentric 

 lamellae ; ventral interarea high, curved apsacline, pseudodeltidium long, arched, 

 foramen submesothyridid, dorsal interarea planar hypercline, chilidium small arch- 

 ing over the dorsal ends of high, thick, narrowly divergent chilidial plates ; shell 

 impunctate. 



Ventral interior with flat semi-oval teeth embedded in thick secondary shell, 

 dental plates absent, ventral muscle field indistinct but apparently not extending 

 much beyond delthyrial cavity ; mantle canal system obscure except for subparallel 



