266 CONFERENCE ON PALEOZOIC PALEOGEOGRAPHY 



been dredged in the warm waters off the Canaries in 50 to 65 fathoms, 

 in the cool waters off the Azores in 2,200 fathoms, in the cold waters off 

 Ireland at 1,443 fathoms, and in Davis Strait at 1,750 fathoms. While 

 temperature does now somewhat restrict the geographic and bathymetric 

 range of the rhynchonellids, still the majority seem to prefer the cooler 

 and deeper waters. This the writer believes to be a modern adaptation 

 that has come about since the Jurassic. 



In regard to the geographic distribution, some of the species are locally 

 restricted, others range through several provinces, and one {H. psittacea) 

 is found throughout the greater part of the northern hemisphere. 



The conclusion derived from the living rhynchonellids, therefore, is 

 that they give no satisfactory guidance as to the bathymetric distribution 

 of the fossil forms. In regard to their geographic occurrence, the Paleo- 

 zoic distribution is very much like that of the living forms, and very 

 little safe guidance is therefore to be derived from them as provincial 

 indicators. Extinct species of the same genus may have local or very 

 wide distribution, may be restricted to a geologic zone of but a few feet 

 in thickness or range through the greater part of a period (Devonic), 

 but sometimes a single species will have a limited time range and yet be 

 distributed over the entire North American continent (Rhynchotrema 

 capax) . 



The terebratulids are in greatest abundance specifically and numeri- 

 cally in the shallower waters. Five occur between the tides and 76 out 

 of the 112 forms, or 67 per cent, live in waters less than 600 feet in 

 depth. The largest of all living species is Magellania venosa, growing to 

 3l^ inches in length and found in abundance in Magellan Straits at 

 depths varying from 50 to 480 feet. 



SHELL CHARACTERS OF DEEP-WATER SPECIES 



The 29 deep-water and abyssal species are all thin shelled, often very 

 fragile, gray or light yellowish in color, more or less transparent and 

 generally small in size. There are, however, large species in the greater 

 depths, but none in the abysses. Such are Dyscolia wyvillii (385-845 

 fathoms), with a length of 2.5 inches; Liothyrina suhquadrata (500-- 

 600), with a length of 1.1 inches, and L. sphenoidea (215-1,090), with a 

 length of 1.2 inches. 



Geologic History of the living Brachiopods 



We will next examine into the geologic range of the 33 living genera 

 of brachiopods. There are at least 23 having fossil representation, and 



