Cretaceous Fossils in Eocene of Maryland. — Bagg. 373 
South Carolina where it is said to be found in the Tertiary, 
but he then adds: "This I think extremely doubtful as it is 
never found above the Middle Marl beds of New Jersey." 
We cannot suppose that this species of brachiopod, so 
abundant in a zone of the Rancocas marl bed disappeared at 
the close of this stage and reappeared again in the Eocene. 
Nicholson in his Manual says: "There is no record of the 
reappearance of any fossil after it has once disappeared. 
There are plenty of cases in which a species seemingly dis- 
appears in a particular set of rocks to reappear in some higher 
and later set of rocks in the same region, whilst its remains 
are wanting in all the intermediate deposits of the area. It 
also often occurs that the species, having disappeared in one 
region, is found in deposits of later age in another area." 
We may therefore infer that if the organism is lacking in in- 
termediate deposits either the forms migrated to another 
region at that particular period or else the conditions were not 
favorable for their preservation. 
The study of the migration of organisms which in the past 
have inhabited the earth has never been adequately worked 
out save in a few instances, but such a study would prove ex- 
tremely interesting and profitable. That such migrations did 
occur and on a wide scale is strikingly shown by such brach- 
iopods as Productus semireticulatus and Streptorhynchus 
crenestria, which occur in the lower Cambrian rocks of Eur- 
ope, Australia. India. China and North America. 
In other words, these forms under consideration must have 
migrated from place to place unless we suppose that they had 
a universal distribution in a universal ocean, a condition 
hardly probable. Inasmuch as sessile forms cannot migrate 
and because brachiopoda in adult life are attached, the mi- 
gration of this group must have occurred when the forms 
were in their larval stage during which they possess the power 
of rapid locomotion. Any geological barrier might, however, 
have prevented a migration even of free-swimming forms into 
closely adjoining areas suitable for their existence. A cold 
current of water might act as such a l)an-ier. We nmst also 
bear in mind that not all synchronous (lei^osits disclose the 
