ORDER in PROTREMATA 315 



apex and muscular area, and usually ventrally cemented. Carboniferous; 

 North and South America, Europe, India, and Russia. 



Streptorhynch-us, King. Very much like Derln/n, but without the ventral 

 septum. Beak acute, incurved, or distorted. Carboniferous and Permian ; 

 America, .Europe, and India. 



Meekella, White and St. John. Very biconvex shells, with the teeth of the 

 ventral valve supported by septiform dental lamellae, which reach to the 

 bottom of the umbonal cavity, and extend forward for one-third or one-half 

 the length of the shell. Surface of valves with coarse costae and fine radiat- 

 ing, often plumose, striae. Upper Carboniferous; North America, Russia, 

 India, and China. 



Triplecia, Hall (Dicraniscus, Hall). Trilobate, unequally biconvex, short- 

 hinged shells. Cardinal process long, erect, and bifurcate. Surface of valves 

 usually smooth, but sometimes striated radially. Ordovician and Silurian ; 

 North America, England, and Bohemia. 



Mimulus, Barrande. Like Triplecia, but with the median fold on the 

 ventral valve. No external evidence of a deltidium. Silurian ; Bohemia and 

 North America. 



Streptis, Davidson. Like Triplecia, but biconvex and bilaterally unsym- 

 metrical. Exterior with lamellar concentric shell expansions. Silurian ; 

 England, Gottland, and North America. 



1 Orthidium, Hall and Clarke. Ordovician ; North America. 



Family 5. Thecidiidae. Gray. 



Cemented Strophomenacea, in which the interior of the shell is impressed with 

 variously indented brachial furrows. Carboniferous to Recent. 



This family was formerly associated with the Terebratulidae. Beecher has shown, 

 however, that brachial supports are wanting, and that a true deltidium is present. 



Lyttonia, Waagen (Leptodus, Kayser). Very large, highly inequivalved, 

 irregular shells, frequently with 

 broad lateral expansions. Numer- 

 ous, laterally directed, brachial 

 ridges in the ventral valve, with 

 corresponding divergent grooves 

 in the median region of the dorsal 

 valve. Carboniferous ; China and 

 India. 



Oldhamia, Waagen (Fig. 523). 

 Differs from Lyttonia in that the 

 ventral valve is sub-hemispherical 

 with the incurved apex covered 



i 11 ., r> 77 7 Oldhamia decipiens, \\aagen. Prouuctus Limestone; Salt 



by a callosity, as in Bellerophon. Range, East India. A, B, Interior of ventral and dorsal 



Carboniferous ; India and China. valves> res i> ective] y < after Waa g e >- 



Thecidea, Def ranee (Thecidium, Sowerby), (Fig. 524). Dorsal brachial 

 impressions with three pairs of symmetrical lobes, radially directed. Cretaceous. 



Thecidea and the following genera of the family Thecidiidae comprise for the most 

 part small, sometimes extremely minute forms, represented from the Trias to the 

 present day ; the climax of diversity occurred in the Cretaceous. 



