ORDER III PROTREMATA 315 
apex and muscular area, and usually ventrally cemented. Carboniferous ; 
North and South America, Europe, India, and Russia. 
Streptorhynchus, King. Very much iike Derbya, 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 Tvriplecia, but biconvex and bilaterally unsym- 
metrical. Exterior with lamellar concentric shell expansions. Silurian ; 
England, Gottland, and North America. 
? Orthidiwm, 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 pi Ba! aniline oe 
by a callosity, as in Bellerophon. Races cnt Aer itatecier of enteab.and (doreal 
Carboniferous ; India and China, #!¥® spectively (after Waagen). 
Thecidea, Defrance (Lhecidium, Sowerby), (Fig. 524). Dorsal brachial 
impressions with three pairs of symmetrical lobes, radially directed. Cretaceous. 

Fig. 523. 
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. 
