BRACHIOPODA. 271 



plecia ?) apiculata and A. (T. ?) incerta, the cardinal area is remarkably high and 

 erect, and the median fold is on the pedicle-valve instead of on the brachial 

 valve, as in the typical species. Should these prove genuine Triplecias, the 

 reversal of the relative convexity of the valves will be an occurrence similar 

 to that observed in the species of Mimulus. The species Produda (T. ?) monili- 

 fera appears to be a normally symmetrical, or slightly unsymmetrical (see plate 

 XXV, fig. 5) Streptis, and the T. ? Graya, a form with an erect cardinal area 

 and a strongly radiate surface ornamentation. In one of these radiated species, 

 the Orthis spirifer aides, McCoy, from the Upper Llandeilo and Caradoc, the ex- 

 istence of the peculiar articulating apophyses of Triplegia has been demon- 

 strated. (See Plate XI a, figs. 10, 11.) This species was referred to Triplegia 

 with doubt by Mr. Davidson, but the certainty of its being congeneric with 

 T. extans in these important features, opens the genus for the present to the 

 reception of similar radiated shells. Another excellent representative of this 

 type of structure is the Orthisina cava, Barrande,* from the Etage D.^. 



Leaving out of consideration the Camarella calcifera, Billings, and the Triple- 

 da primordialis, Whitfield, the earliest known representative of this genus in 

 American faunas, appears to have been such a radiated form, T. radiata, Whit- 

 field, from the Calciferous fauna at Beekmantown, N. Y. In the fauna of the 

 Trenton group are T. extans, T. cuspidata and T. nucleus ; the only other member 

 of the genus known in this country being the T. Ortoni, from the Clinton fauna 

 of Ohio. 



* SystSme Sihirien du Centre de la Bolieme, vol. v, pi. 59, figs, iv, 1-7. 



