254 GEOGKAPHICAL AND GEOLOGICAL DISTRIBUTION. 



others. A marked exception to the above law is furnished by the 

 type form of Terebratula vitrea, of the Mediterranean and adjacent 

 seas, whose range varies between five and fourteen hundred and 

 fifty-six fathoms. The greatest depth whence any brachiopod has 

 been obtained is twenty-nine hundred fathoms (Terebratula "Wy- 

 villii). 



Of about one hundred and seven species with whose range we 

 are acquainted, fifty-seven, or more than one-half, are restricted to 

 depths of under one hundred fathoms, and of these a large propor- 

 tion properly belong to the shore-line, or to a zone of from five to 

 fifteen fathoms water. There are twenty-one species (or varieties) 

 whose range extends to, or above, five hundred fathoms; ten with 

 a range of one thousand and upward ; and three with two thousand. 

 These facts indicate, as Mr. Davidson has pointed out, that the 

 " greater bulk of known species live at comparatively small or mod- 

 erate depths," and that " Brachiopoda are specifically rare at depths 

 varying from five hundred to twenty- nine hundred fathoms." As 

 to numerical distribution we find, as the result of the " Challenger's " 

 explorations, that in ninety-nine dredgings, taken in water of from 

 one to five hundred fathoms, Brachiopoda were brought to the sur- 

 face about twenty-three times ; in thirty dredgings, of five hundred 

 to one thousand fathoms, four times; in ninety-four dredgings, 

 of one thousand to two thousand fathoms, nine times; and in one 

 hundred and seventy-six dredgings, of two to three thousand fath- 

 oms, only six times. It is seen here, therefore, that the numeric 

 diminution keeps pace with the specific, and that practically the 

 Brachiopoda cease to abound in depths exceeding five hundred 

 to six hundred fathoms. The proper appreciation of these facts be- 

 comes of prime importance when discussing, the nature of geologi- 

 cal deposits containing brachiopod remains. 



In respect of numerical development and broad distribution, 

 both in time and space, the Brachiopoda constitute, for the geolo- 

 gist, the most important landmark in the determination of his hori- 

 zons. Beginning with the very earliest fossiliferous formation, the 

 Cambrian, and there already in nearly the bottom bed (St. David's), 

 they continue throughout all time, and if, during the more recent 

 geological periods, they have suffered rapid diminution, both in 

 actual numbers and the variety of forms, they are still sufficiently 

 abundant and varied to indicate that a long period must elapse 



