BRACHIOPODA. 273 



valve, while the other, M. contrarius, has it on the brachial valve! Again, all 

 the Bohemian species appear to be bilaterally symmetrical shells, while the 

 American Mimulus Waldronensis, which has the fold on the brachial valve, is 

 always unsymmetrical in its mature condition, and may thus bear pretty much 

 the same relation to M. contrarius as Steeptis does to Triplegia. 



A most striking feature in these shells is the character of the cardinal area, 

 which usually in Bohemian species, and invariably in the American form, is 

 smooth and without any superficial evidence of the delthyrium. . In fact this 

 opening is closed by a thin Hat plate, which may be sometimes broken away or 

 be absent from other causes, as appears from some of Barrande's figures.* It 

 has been shownf that in a very early growth-stage in M. Waldronensis the del- 

 thyrium is open and its apical termination circular. While the delthyrium 

 becomes closed, apparently by lateral accretion to the walls of the fissure, this 

 foramen is retained until maturity .J In the interior of this valve in M. 

 Waldronensis there are no dental lamellae, in which respect the genus diflfers 

 from Triplegia. The brachial valve bears an erect, deeply bifurcated cardinal 

 process, of quite similar character to that in Tripleda extans. The other interior 

 features of Mimulus are still unknown, but those described are sufficient to 

 demonstrate the close alliance of these shells with the streptorhynchoids. All 

 the Bohemian species are from the Etage E.^, which is the equivalent of the 

 Niagara fauna. 



* See Barrande, pi. 9, figs, iv, B, g. 



t Beecher and Clarke, op. cit. 



I It will be observed that in Triplegia, Steeptis and Meekella, the median convex ridge of the delti- 

 dium is very narrow, occupying but a small part of the area between the dental plates. The obliteration 

 of this low ridge would show a character similar to that of the deltidium in Mimdlus. 



