56 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



short; delthyrium small, trigonal, constricted by pair of conjunct 

 deltidial plates, leaving small elongate elliptical foramen only slightly 

 penetrating apex of beak ; posterolateral edges of valve slightly over- 

 lapped by narrow flange of brachial valve. 



Brachial valve slightly flatter, also recurved on crests of plications ; 

 beak bluntly pointed, apex within pedicle valve beneath deltidial plates. 



Pedicle valve interior with dental plates forming deep spondylium, 

 meeting floor in apex of valve for 1 mm. or less, then combining to 

 form low median septum duplex, elevating spondylium slightly for 

 remainder of length; spondylium continuing anterior to near mid- 

 length, septum extending somewhat beyond midlength. 



Brachial valve interior with short concave hinge plate, depressed 

 slightly to meet top of intercamarophorial plate ; camarophorium be- 

 ginning near apex of valve as median septum with narrow lateral pro- 

 tuberances, widening anteriorly, entirely separate from hinge plate 

 except for connection through intercamarophorial plate, curving 

 ventrally on increasingly high septum, trough nearly flat in posterior, 

 becoming shallowly concave toward anterior, extending beyond mid- 

 length of valve, continuing beyond anterior edge of supporting septum ; 

 crura shorter than camarophorium. 



Holotype.— Belanski, 1928, pi. 4, figs. 10-14, No. 10 in Belanski 

 Coll. ; illustrated specimen : USNM 71027. 



Occurrence. — Shell Rock Limestone of Thomas (1924) at Nora 

 Springs, Iowa. 



Age. — Early Late Devonian. 



Table 6. — Measurements of Belanski paratype specimens of Atribonium pau- 

 perum from the Shell Rock Limestone of Thomas (1924) Nora Springs, Iowa 



Measurements, in millimeters 



Comparisons. — Atribonium pauperum is characterized by its strong 

 and sharp plications. They are too deep to be termed costae; they 

 greatly fold both valves as well as the commissure. No other species of 

 Atribonium is as strongly folded. 



Discussion. — This species is described and illustrated to show the 

 extent of variation within the genus. Despite its extreme costation or 

 plication, it retains the external characters of short beak with small, 

 open foramen, only slightly overlapping posterolateral valve edges, and 



