322 
PALEONTOLOGY OF NEW YORK. 
Elkania and Dinobolus (p. 28, plates iii, iv h). 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 Lingulacea 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. Lindstmmi bear long submarginal 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 been 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 been 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 makin’.” It had, nevertheless, 
been the intention to summarize, in a tabulated form, at the close of this woi’k, the broader relations of 
the genera discussed, not with any intention of introducing a series of new taxonomic terms, but to express 
succinctly these interi’elations as they appear upon a review of the whole field of research. Such a table 
will be found at the close of this chapter. 
