68 SHELVE DISTRICT 



Family ALIMBELLIDAE Andreeva i960 emended A. W. 



Diagnosis. Biconvex, smooth to capillate, uniplicate orthaceans with well- 

 developed curved interareas and open delthyrium and notothyrium ; ventral muscle 

 field impressed on pseudospondylium, dental plates absent at least in adult shells ; 

 blade-like cardinal process and strong brachiophores supported by variably devel- 

 oped notothyrial platform ; ventral mantle canal pattern saccate, dorsal digitate. 



Discussion. This family was erected by Andreeva (i960 : 292) to include two 

 Tremadocian genera from the Urals, Alimbella Andreeva i960 and Medesia An- 

 dreeva i960 which were considered by her to be aberrant porambonitaceans, a 

 conclusion accepted by Biernat (in Williams et al. 1965 : H530), in her review of the 

 Syntrophiidina for the Treatise on Invertebrate Paleontology. In her discussion of 

 the affinities of the Russian genera, Andreeva conceded that the morphology of the 

 brachial valves is typically 'orthoid, similar to Platystrophia (i960 : 292) but believed 

 the organization of the pedicle valve to be so decisively porambonitacean as to 

 preclude any connection with the orthaceans. It is difficult to understand how 

 Andreeva came to this conclusion unless she had attached an overriding importance 

 to the uniplicate condition of the shell which is certainly much more characteristic 

 of the porambonitaceans than the orthaceans. Yet, if the ventral sulcus is ignored, 

 the pedicle valves of both genera are immediately seen to be unexceptionally 

 orthacean in the strong development of the interareas, the arrangement of the ventral 

 muscle fields, and above all in the saccate mantle canal systems, with the proximal 

 parts of the vascula media disposed medially in a manner which is unknown among 

 the digitate porambonitaceans. The pseudospondylium is admittedly a rare feature 

 among orthaceans but is known to have developed independently in several articulate 

 groups and is, in any event, as well defined in the orthid Glossorthis as it is among the 

 alimbellids. 



All other features displayed by the Russian genera, such as the cardinal process 

 and lack of brachiophore supports, are much more, but not exclusively, characteristic 

 of the orthaceans rather than the porambonitaceans. It therefore seems reasonable to 

 transfer the family to the Orthacea and to regard the stocks assigned to it as bearing 

 the same relationship to the Orthidae as the Platystrophiinae do to the Plectorthidae 

 and Finkelnburgiidae. 



ASTRABORTHIS gen. nov. 



Name. An orthacean shaped like a saddle (Gk. aoTpafSri) . 



Diagnosis. Subcircular, biconvex, uniplicate shells with a deeply sulcate pedicle 

 valve ornamented by coarse angular costellae ; ventral interarea curved apsacline 

 with wide, open delthyrium ; dorsal interarea curved anacline with wide, open 

 notothyrium ; shell probably impunctate. 



Ventral interior with simple rounded teeth ankylosed to a pseudospondylium by 

 solid deposits of secondary shell in adult valves, which may mask short receding 

 dental plates, pedicle callist absent ; broad, medially divided adductor scars and 



