BRACHIOPODA. 
47 
is accompanied by certain peculiar phenomena. According to our present 
knowledge its first appearance is in the little Lakhmina, from the primordial 
“ Obolus-beds,” in the Salt-Range of India, but in American faunas, where the 
development of the group is best exemplified, it is first met in Dinobolus (in 
external features the most oboloid form of the group), in the later faunas of the 
Lower Silurian; Black River, Trenton, Galena. Thereupon follow in the still 
later fauna of the Hudson group the more pronounced linguloid genera, Lingulops 
and Lingulasma. Not, however, until the introduction of the Niagara or Wenlock 
fauna does the entire group, with the exclusion of the inceptive linguloid forms, 
reach its culmination in specific and individual development. The magnesian 
deposits constituting the Guelph and Galt limestones of the Province of Ontario, 
and the Niagara limestones of the interior of the United States, seem to have 
been accumulated under conditions favorable for the rapid increase of these 
animals, and yet, notwithstanding that in the limestones of the Niagara of New 
York, the Aymestry and Wenlock of England, and the Etage E-e 2 , in Bohemia, 
these fossils have been found but sparingly, the equivalent faunas of the Island 
of Gotland, involved in essentially the same physical environment, produced 
certain of the genera in vast numbers. This was the period of the culmination 
of the Trimerellids, irrespective of their surroundings. With the disappear¬ 
ance of this fauna, the platform-bearing brachiopods virtually became extinct, 
and we have as yet no trace whatever of the occurrence of this peculiar feature 
at any later date, or in any other group of these animals.* 
In the genus Trimerella only, do we meet with a constant development of the 
platform in both valves, as a compound vaulted arch ; in the other genera it is 
a solid plate, always showing a tendency to excavation on its more or less con¬ 
cave anterior walls, while Dinobolus furnishes frequent instances of this tendency 
being carried further toward the development attained in Trimerella, though 
its vaults, when developed, are small, narrow and constricted, by no means bear¬ 
ing the same dimensional proportion to the platform as in the latter genus. 
* In some genera of the articulate brachiopods the anterior and lateral edges of the muscular area is 
at times conspicuously elevated, for example, frequently in Lept^na, Strophomena, Strophobonta, and 
Streptoriiynchus. To what extent this elevation is of the same nature and due to the same causes as the 
platforms of the'Trimerellids, is yet to be determined. 
