68 
ORGANIC DEPENDENCE AND DISEASE 
fauna of the Silurian age. There seems no good reason for 
not interpreting* this occurrence, even though it may he iso- 
lated, as a definite and purposeful fixation. To regard it as 
casual would be sophistication. With these preliminary 
evidences then we find the snail-crinoid adjustment well 
under way very early and clearly developing from a habit 
of visitation into a set adjustment to vitiation and para- 
sitism. 
As we enter the Devonian or next succeeding period of 
geological time, we are confronted at once by an enormous 
development of the holostomatous shells 
of the limpet group, known as Capulus 
or Platyceras, with various generic 
allies. They are all very closely allied 
to the species we have already re- 
ferred to and have the same inheritance. 
So abundant are these Devonian limpets 
and so great their variety that it has 
more than once been proposed to desig- 
nate the early Devonian age by the name 
Capulian or Platjmeras stage. In a sud- 
den outburst of evolution these mollusks 
here reached their climax. Every stu- 
dent of these shells recognizes the diffi- 
culty of assigning specimens of them to 
well-defined species, because the thin- 
ness of the shell mouth and its easy 
adjustability in fixation to the irregulari- 
ties of the surface to which they may 
attach themselves have caused many 
differences of expression. Attachment 
was their habit and we may expect to find them fixed 
to various sorts of objects, organic and inorganic, when- 
ever they have been caught alive or in a cemented condition 
by the burying sediments. We must then scrutinize every 
Fig. 54. Views of the 
Silurian crinoid Mar- 
sipocrinus coelatus 
(Phillips) sur- 
mo anted by, and 
partly enclosing an 
attached snail. Wen- 
lock limestone, Dud- 
ley, England. (Cour- 
tesy of Mr. Frank 
Springer. ) 
