of the YorTctown and Duplin Formations. 307 



53*5 per cent ; the nine species of Scaphopoda and the single 

 chiton (Amphinenra) bringing the total up to 802 forms. One 

 hundred and eighty-nine, or 23*6 per cent, of these are new to 

 the literature. 



The species persisting to the Recent fauna and upon which 

 the deductions concerning the environmental conditions are 

 chiefly based, number one hundred and eighty-five, of which 

 62*5 per cent are univalves and 364 per cent bivalves. The 

 larger percentage of Gastropoda is possibly due to the more 

 uniformly arenaceous character of the deposits of the Wac- 

 camaw and Duplin, the formations from which the most pro- 

 lific faunas have been derived and which were laid down, 

 apparently, under conditions more favorable to the univalve 

 than to the mud-loving bivalve. 



Of the one hundred and eighty-five species and subspecies 

 of Tertiary mollusca represented in the recent seas, fifty-six, 

 according to the latest check-lists, which are, of course, nothing 

 more than the latest approximations to the truth, range both 

 to the northward and the southward of the Virginia-North 

 Carolina coast ; eighty-four from North Carolina, southward ; 

 nine from Virginia, northward ; while thirty-six have not been 

 reported north of Florida. It is very possible, however, that 

 some of these extra-limital species did not inhabit the area in 

 which their shells were buried. Most of them are very small, 

 and represented, in many cases, by a single individual, which 

 may easily have furnished food for a Tertiary fish in the 

 Antillean region, and later been disgorged in his journey up 

 the Gulf Stream. In fact, Conus proteus Hwass and Petalo- 

 conchus nigracans Dall are the only forms not occurring off 

 the Virginia and North Carolina coast to-day which are not 

 rare in the late Tertiary faunas. Then, too, the northern limit 

 of extension of a fauna is much less sharply defined than the 

 southern, since forms which push southward can often repro- 

 duce under the new conditions while the larvae of a form which 

 has extended its range to the northward are frequently exter- 

 minated by a drop of only a couple of degrees in temperature. 



The relative abundance of a race is, within certain limits, a 

 much surer guide to the conditions under which it existed than 

 a check-list of the species occurring within that area. Thus in 

 the Yorktown formation, the Nuculas, Ledas, and Yoldias, all of 

 them genera which flourish in the cooler waters, are very abund- 

 ant ; Glycimeris americana deFrance and Glycimerisjpectinata 

 Gmelin, species w r hich range to-day from Hatteras southward 

 throughout the W.est Indies, although listed in the Yorktown 

 faunas are inconspicuous elements, while in the faunas south of 

 Hatteras they occupy a major position. The distribution of 

 the Pectens and the distribution and abundance of the Astartes 



