306 MOLLUSCOIDEA BRACHIOPODA SUB-KINGDOM v 



Elkania, Ford (Billingsia, Ford). Like Obolella, but with a small platform 

 in the dorsal shell. North America. 



Obolus, Eichwald ( Ungula, Pander ; Ungulites, Bronn ; Aulonotreta, Kutorga ; 

 Acritis, Schmidtia, Volborth), (Fig. 506). Much like Obolella externally, but 

 larger and with more prominent cardinal areas in both valves. Ventral valve 

 with a median septum extending to about the centre of the valve. 0. Appolinis, 

 Eichwald, occurs in vast numbers in the Unguliten-Sandstein of Russia. 



Neobolus, Waagen ; India. Botsfordia, Matthew New Brunswick. Spondyl- 

 obolus, M'Coy (Fig. 507). Ireland. 



Family 3. Trimerellidae. Davidson and King. 



Large, thick-shelled, inequivalved Obolacea, with the ventral cardinal area usually 

 very prominent, triangular, and tranversely striated. Adjuster and anterior adductor 

 muscles elevated upon solid or excavated platforms. Cambrian to Silurian. 



Dinobolus, Hall (Conradia, Hall; Obolellina, Billings). Cardinal area not so 

 prominent as in the other genera of this family. Platform small, with abruptly 

 conical vaults. Ordovician and Silurian ; North America, Great Britain, 

 Bohemia, Gottland, and Esthonia. 



Monomerella, Billings. Similar to Trimerella, with well-developed platforms 



FIG. 508. 



Trimerella Lindstroemi, Ball sp. Silurian ; Gottland. x i/o. A, Shell seen from the dorsal side. B, C, In- 

 terior of dorsal and ventral valves, respectively. D, Cast. (A and B a/ter Davidson ; Cand D after Lindstrbm.) 



in both valves ; that of the dorsal valve, however, but slightly excavated. 

 Silurian ; North America, Gottland, and Livonia. 



Trimerella, Billings (Gotlandia, Dall), (Fig. 508). Platforms long, narrow, 

 well -developed, and doubly vaulted. Dorsal beak often thickened into a 

 prominent apophysis extending against the cardinal slope of the ventral valve. 

 Silurian ; North America, Gottland, and Faro. 



Pihtiiobolus, Hall. Silurian ; North America. (?) Lahkmina, Oehlert (David- 

 sonella, Waagen). Lower Cambrian ; India. 



Superfamily 2. LINGULACEA. Waagen. 



Elongate, thin-shelled, burrowing, derived Atremata, with a more or less long, 

 worm-like, tubular, flexible pedicle. Cambrian to Recent. 



Family 1. Lingulellidae. Schuchert. 



Spatulate, inequivalved Lingulacea, structurally intermediate between the Obolidae 

 and Lingulidae. Cambrian and Ordovician. 



