INTRODUCTION^. ll 



Mexico, or California. These forms are very interesting, aS most of the 

 species originally common to both have developed special modifications 

 since the separation of the two oceans, so as to be entitled to separate 

 specific names. 



A column (West. Am.) is devoted to recording those found on both 

 sides of the continent yet which still remain essentially nnchanged, and 

 another. (Eur.) to those whose range extends to European shores. 



Another column is devoted to the soutliern extreme limit (as far as 

 known) of the species enumerated in the catalogue, corresponding on 

 the south to the column for northern limit on the north. Many Antil- 

 leau species extend on the Brazilian coast far south of Cape San Roque, 

 but our records for this region are very imperfect, and many of the 

 items in this column are due to the data obtained by the U. S. Fish 

 Commission steamer Albatross on her voyage from the Chesapeake Bay 

 around to California via the Straits of Magellan only a year ago. 



A column records the oldest known appearance of a species in geo- 

 logical time. This column is very imperfect and inadequate to express 

 the real state of the case, since many of our recent species have been 

 described from our southern tertiaries under other names, and the du- 

 plication thus occasioned, except in a. comparatively small number of 

 species, still remains to be worked out. It was thought well, however, 

 to make a beginning in the matter in this instance. 



This completes our description of the table, which will enable any one 

 to use the latter intelligently and without misconception. 



In making entries in the columns showing distribution an asterisk 

 shows that the species is known from that region from the shores, either 

 picked up on the beach or found living between high water and fifty 

 fathoms, or that the depth it inhabits is not known but is supposed to 

 be small. In cases where the species is recorded from the archibenthal 

 area only, say 50 to 800 fathoms, its presence is indicated by a dagger 

 point in the column. When both an asterisk and a dagger point are 

 found in a single column the species is supposed to occur, or is recorded 

 as obtained, both in shallow and in deep water, within the limits of 

 that region or district. Many southern species, found in the cool 

 water of the deeps in the south, approach the surface in the cooler 

 surface waters of their northern range. Vice versa, we find northern 

 littoral species seeking the deeps as they approach the limits of their 

 southern range. A glance at the columns frequently will illustrate 

 these facts. 



The data from which the tables which form the bulk of this pub- 

 lication have been compiled are chiefly comprised in the collections 

 of ttie U. S. National Museum, the Museum of Comparative Zoology 

 in Cambridge, Mass., and the publications of the writer on these col- 

 lections. The works in which detailed information has been chiefly 

 sought are specified on another page, but the most important for this 

 purpose has been the Report on the Blake Brachiopoda, Pelecypoda, 



