38 Meek and Worthen on Paleozoic Crinoidea. 
shaly matter too firmly adherent to prevent the line of contact 
between the shell and Crinoid to be clearly seen, the sinuosities of 
the lip of the former closely conform to the irregular nodose 
surface of the latter. Owing to the fact that in some cases the 
shell has evidently been forced by accidental pressure against 
the surface of the Crinoid, so as to become somewhat crushed, 
this adaptation is not always so clearly evident ; but in most 
cases it is more or less visible, while in some it is strikingly 
manifest. In one instance of a Platycrinus now before us, 
with a Platyceras attached, as usual, to its side, between the 
arm-bases of two of its adjacent rays, and of rather larger size 
than those usually found attached to this species, the adapta- 
tion of the irregularities of its lip, so as to receive the little 
nodes and other prominence of the Crinoid, is so clearly man- 
ifest that a moment’s examination must satisfy any one that 
the shell must have grown there. Being, as we stated, a lar- 
ger individual than we usually see so situated, it not only occu- 
pies the whole of the interradial or anal space to which it is 
attached, but its lateral margins on each side coming in con- 
tact with the arm-bases of the Crinoid, as the shell increased 
in size, had formed on either side a profound sinus in its lip 
Jor the reception of these arms. These sinuses are not only in 
precisely the proper places, but of exactly the proper size and 
Jorm to receive the adjacent arm on each side; the entire ad- 
justment being so exact, that it seems scarcely possible that 
the shell could have been removed during the life of both ani- 
mals, and after the mollusk had attained its present size, with- 
out either breaking its lip or breaking off the arms of the 
Crinoid, Unfortunately, in clearing away the rather hard ar- 
gillaceous matrix, before the arrangement of the parts was 
clearly comprehended, these arms were broken away, but their 
stumps are still seen protruding from the sinuses, which are so 
deep as almost to present the appearance of isolated perfora- 
tions, though it is evident, on a careful examination, that they 
are only deep emarginations extending up from the edge of the 
p- 
In looking at the sides of this Platyceras which has the 
form of a very slightly arched cone,* and stands out nearly at 
right angles to the side of the Crinoid, it is easy to see, from 
abrupt curves in the lines of growth, along up its sides, on a 
line above the sinuses mentioned, that these sinuses com- 
menced forming abruptly at points about half way up ; and on 
measuring across between these points with a pair of dividers, 
* It being the common species of Platyceras that is usually found attached to 
this Platycrinus, 
