BRACHIOPODA. 
163 
clearly developed in the brachial as well as in the pedicle-valve, a large cen¬ 
tral (umbonal ?) muscular scar, and conspicuous, undifferentiated lateral scars. 
The fact that the culmination of the obolelloid type was attained in faunas 
where Lingula is not yet known, fortifies the view that Lingula has been directly 
derived from this source. In Lingulella and Lingulepis may be found impor¬ 
tant connecting links, in which the external form of Lingula is associated with 
the muscular arrangement and the narrow pedicle-slit of the obolelloids. These 
two genera are also forerunners of Lingula. 
By the attainment of the linguloid ensemble, a combination of characters was 
produced which was capable of adapting itself to all conceivable changes in its 
surroundings to an extent never recurring in the organic world. But while the 
development along the linguloid line has continued, as we believe, from early 
Silurian to the present time, modifications of this type were not infrequent. 
From Lingula we may depart in many directions. A gradual increase in the 
secretion of testaceous matter about the insertions of the muscular and parietal 
bands, carries eventually to an extreme the development of median and lateral 
septa. One phase of this extreme, in which the septa are alike in both valves, 
is attained, in Dignomia, as early as the Devonian; another, with some diversity 
in the septa of the opposite valves (and quite plausibly in the direct line of 
derivation from Dignomia), appears first in the Mesozoic, and has continued to 
the present in Glottidia. The numerous forms in which the septa are more 
or less developed without attaining the condition in Dignomia or Glottidia 
are better left within the proper limits of Lingula. 
Again, the elevation of the anterior edges of the testaceous deposits about 
the bases of the central and lateral muscles, probably due, as we have elsewhere 
suggested (see pages 46-55), to displacement of, and pressure against the liver, 
has induced first, the thickening of the entire area of muscular implantation, 
followed by the gradual excavation of this solid plate and the formation of a 
more or less vaulted platform. Thus was begun the line of variations con¬ 
secutive through Lingulops and Lingulasma, the extreme of which is reached 
in Trimerella. In the later Silurian sea the conditions appear to have been 
especially favorable to the rapid development of these platform-bearing species, 
