BRACHIOPODA. 58 



faunas of still earlier date. IIhinobolus has the aspect of a degenerate Triui- 

 erellid, marked by the general ol)solescence of the characteristic features of the 

 group. In L.-VKHMiNA, we have evidence that the platforms attained a con- 

 siderable development before the introduction of Silurian faunas. Features, 

 which in American faunas appear to have developed slowly, and whose different 

 stages can be satisfactorily traced, seem to have been specialized with great 

 rapidity in this obscure fossil. It is not necessary to assume that this fossil 

 from a distant fauna, now regarded as of primordial age,* will materially 

 modify the conclusions expressed in the foregoing diagram, derived from the 

 studj' of American faunas. 



Having indicated that the inception of the platform was probably due to aug- 

 mented muscular energy and concomitant increased secretion of muscular 

 fulcra, we may revert to the consideration of the function of this organ in its 

 highest development, and to the inquiry, whether this attainment may have 

 been acquired alone by the constant action of the forces named, or has been 

 aided to this result by the accessory action of other organs. 



Whether or not the platform be hollow as in Trimerella and LAKHMrNA, and 

 sometimes in Dinobolus, or solid, as usually in Dinobolus, and always in the 

 other genera, there is invai'iably a manifest tendency toward its excavation. The 

 anterior walls are always concave ; in Monomerella the concavity of the plate 

 is deep, the anterior moiety being turned with a steep slope toward the cavity 

 of the shell ; furthermore, the shell itself is much depressed beneath this 

 projecting wall and on either side of the anterior median septum into which 

 the platform is continued. The same features are seen in the brachial valve 

 throughout these genera.f Even in Rhinobolus, all that is lett of the platform 

 is turned upward and inward at this sharp angle. In the pedicle-valve, how- 

 ever, this inclination of the platform, the excavation of its anterior walls and 



* See iiagfe 29. 



t Mr. TJlrich describes a peculi.ir structure in the jilatform of Lisgula.sm.4. He says: "The cost of the 

 interior which furnished the gutta-percha sriueezes represented by figrs. it and m" (see I'l. II, Hg. 19), "orijj- 

 inally preserved much of the shell and all of that pertaiiiingr to the platform. This was carefully removeil, 

 and during the process it w.ns noticed that the platform consisted of numerous cup-shaped laininie placeil 

 within one anothei-, and so that an open space was left between each and the preceding and succeeding ones " 

 (American Geologist, vol. iii. No. 6, p. 386. ISS9). 



