262 CONFERENCE ON PALEOZOIC PALEOGEOGRAPHY 



line, but that in water less than 90 feet deep there occur about 49, or 

 y 31 per cent, of all living brachiopods. The most conspicuous of these 

 shallow-water forms are of Lingula and Discina, genera that are re- 

 stricted to the littoral region — that is, ranging from the strand-line to a 

 depth of probably not much more than 60 feet. In addition to this, 

 many of the species of lingulids occur in bays and estuaries, indicating 

 that they prefer a habitat more or less freshened by land waters. Dis- 

 cinisca also lives in the littoral region, but apparently never on the 

 strand-line, and no species goes beyond 216 feet of depth. Glottidia 

 commences at lowest tide, and has been taken as far down as 360 feet. 

 Crania is not reported from the tide-line, but begins in 12 feet of water, 

 and extends its bathymetric range to 808 fathoms. Of all the living in- 

 articulate brachiopods, but one is completely habituated to deep water — 

 Pelagodiscus atlanticus — ^which has a range from 200 to 2,425 fathoms. 

 It is also a cold-water species, and its geographic range appears to be 

 now or to have been world-wide, for it is known in the north and medial 

 Atlantic Ocean, the Pacific, and off Australia. In its arm structure it 

 is also very primitive, in that the brachia are not spirally rolled, but 

 "form two simple loops, with no spirals whatever" (Dall, 1907). In 

 other words, the brachia are in the schizolophus stage, as defined by 

 Beecher,^ and therefore do not develop into the more complicated struc- 

 tures seen in most living brachiopods. 



A survey of the geographic distribution of the inarticulate brachiopods 

 also shows that all the littoral and shallow-water species are bound to 

 warm waters, and that hardly any are common to two zoological prov- 

 inces. Furthermore, when the shallow-water and littoral forms are com- 

 pared with those of the deeper, and especially the one species of abyssal 

 waters, we note that the former are decidedly more prolific in numbers, 

 are often considerably larger and always have thicker shells, while the 

 deeper water forms are nearly always smaller and have thin and nearly 

 transparent valves. The smallest form with the thinnest valves is the 

 abyssal Pelagodiscus atlanticus. 



These results are of the greatest value to the paleogeographer, for they. 

 can be successfully applied to the fossil Inarticulata, and through this 

 knowledge one can state positively the depth of water at which the fossil 

 lingulids and discinids lived. Further, they are excellent guides as indi- 

 cators of shorelines, and as such give clear guidance to the paleogeography 

 of any given time. 



The paleontologist finds that the greatest number of large, thick 



3 Beecher : BuU. U. S. Geol. Surv., vol. 87, 1897, p. 108. 



