BRACIUOPODA. 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 gToup 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 Gait 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,, 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 Leptjena, STnoPHOMENA, Stropiiodonta, and 

 Stbeptokhynchds. 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 lie determined. 



