822 PALEONTOLOGY OF NEW YORK. 



Elkania and Dinobolus (p. 28, plates iii, iv6). The chronogeny of the various 

 elements is in full accord with the structural progress along both lines of deriva- 

 tion; a single genus in this series, Lingulops, enduring in an unmodified condi- 

 tion from faunas (Hudson River) antedating the appearance of Trimerella, to 

 those in which Trimerella abounds (Niagara and Guelph dolomites). 



The entire group of linguloid and oboloid genera is bound together, as 

 already shown, by the possession of an unenclosed marginal pedicle. They 

 compose the Mesocaulia or Lingdlacea of Waagen (1883) (Atremata of 

 Beecher, 1891).* 



The leading element in this group, Lingula, attained a static condition in early 

 Silurian faunas ; the oscillations of the type were mainly confined to the preced- 

 ing faunas ; those of later date are but slight departures in a few directions only. 

 The combination termed Lingula having once become fixed, maintained itself 

 with unexampled adjustment to changing conditions, even into the existing seas. 

 Glossina, Dignomia, Barroisella and Tomasina, which represent early deviations 

 along the line of its descent, embody no substantial variations, though the two last 

 named (pp. 62, 65, plate ii) demonstrate the gradual assumption of articulating 

 processes, a tendency which not infrequently makes itself apparent in this group 

 where the pedicle-passage is wholly marginal. It is seen in Spondylobolus, and 

 is sometimes faintly manifested in Obolus and Obolella ; in Trimerella there 

 is occasionally a low cardinal process as shown by Davidson and King, and Got- 

 land specimens of T. Lindstrami bear long subraarginal slotted ridges on the 

 cardinal edges (Lindstrom). This mode of articulation, though not frequently 

 seen in American specimens of Trimerella, is so much like that of Eichwaldia, 

 and the general form of the shells of the two genera is so similar, that there is 



*To ensure {greater freedom of treatment and relief from the embarrassments of an inelastic classifica- 

 tion, the discussions in these volumes have intentionally l)een left free of terms designating taxonomic 

 values higher than genera. By provisionally declining allegiance to any prescribed formulas in classifica- 

 tion, not only has the manner of treatment of the comprehensive material studied boen more natural, but 

 the student will find himself less encumbered with artificial restrictions and freer from collisions with rock- 

 ribbed party-walls, which, to use an old Scotch phrase, "are nane o' God's inakin'." It had, nevertheless, 

 been the intention to summarize, in a tabulated form, at the close of this work, the broader relations of 

 the genera discussed, not with any intention of introducing a series of new taxonomic terms, but to express 

 succinctly these interrelations as they appear upon a review of the whole field of research. Such a table 

 will be found at the close of this chapter. 



