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 Degnomia), 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. 



