RECENT BRACHIOPODA 259 



forms, and these are distributed in 33 genera. Of these about 8 forms 

 are poorly defined, so that there are at least 158 established species of 

 living brachiopods. However, with the refined methods recently intro- 

 duced by Blochmann and Dall, and in the further dredging that is now 

 going on in Antarctic waters, we may expect a considerable increase in 

 the number of species, so that eventually there may be a total of about 

 180 forms. Whatever the additions may be to our knowledge, that in 

 regard to their habitats, bathymetric and geographic range will be 

 slight. We therefore have safe guidance in the living members of the 

 class as to what the bathymetric range and habitats of the fossil forms 

 were. 



The 158 living forms are grouped according to their relationship as 

 follows : Of Inarticulata there are 29 and of Articulata 129. The in- 

 articulate brachiopods are nearly equally distributed among the orders 

 Atremata (15) and Neotremata (14), there being of lingulids 15, dis- 

 cinids 7, and of Crania 7. Of the once wonderfully prolific Paleozoic 

 order Protremata there are but 2 living representatives in Thecidium, 

 a genus that arose in the Cretaceous. They are small forms, and though 

 not rare in the Mediterranean and Antillean regions, are restricted to a 

 depth ranging between 30 and 300 fathoms. Both also occur fossil 

 since the Miocene or Pliocene, and are therefore morphologically static. 

 The Telotremata are the dominant brachiopods, being represented by 15 

 rhynchonellids and "112 terebratulids, and of these the last named had 

 far less differentiation in the Paleozoic. Both stocks are very ancient, 

 the rhynchonellids being as old as the Middle Ordovicic and the tere- 

 bratulids arising early in the Devonic. While neither stock was prolific 

 in genera and species throughout the Paleozoic, both stocks began to 

 evolute in the late Triassic, and in the Upper Jurassic the seas swarmed 

 with a great variety of these animals, and especially of the terebratulids. 

 The decline of the latter began in the Cretaceous and persisted to the 

 Oligocene, when the warm waters of this time seem to have rejuvenated 

 the stock to its present good representation. On the other hand, the 

 rhynchonellids have maintained their generic and specific variation fairly 

 constant since the Siluric, when for the first time the stock was well 

 established. In the Jurassic, however, they were more abundant than 

 at any other time. We learn, therefore, that though these two stocks 

 are very old, they are still morphologically young and plastic, constantly 

 giving rise to new forms and new genera. 



