117 
the articulatine surfaces of joints were concave, striated, and had a small ali- 
mentary canal, they have produced slightly convex impressions distantly re- 
sembling a limpet, and thus probably have led to the assertion of the occur- 
rence of that shell in beds below transition limestone. 
\ 13. Much has been said by the older writers on fossils regarding a petre- 
faction called by them screwstones, which, although at first sight much resem- 
bling a screw, show on examination that the incision around them is not spiral 
but circular. They prove on reference to the columns of the Crinoidea to be 
casts, formed in the alimentary canal, as in a mould, either by a harder 
substance such as chert, which from its want of affinity to the surrounding: 
substance did not incorporate with it, and resisted decomposition after the 
destruction of the column; or by the infiltration of a highly saturated calca- 
reous menstruum which filled the empty cavity, formed laminar spar, and took 
its markings gradually, being prevented by the intervention of the alimentary 
vessel itself from incorporating with the column, and enabled to preserve its- 
form after the decomposition of the column. I have seen casts in chert of the 
round (Actinocrites, &c.) and pentagonal (Cyathocrinites rugosus and Rhodo- 
. crinite) alimentary canal of crinoidal columns, which displayed the effects of 
sphincter contraction, and in which where the cherty matter had infiltrated 
between the radiating adhering surfaces, a thin and elegantly striated rim sur- 
rounded the thickest part of the cast. I also have a fine calcareous cast formed 
in the upper part of the alimentary canal and the abdominal cavity of Apio- 
crinites rotundus, where the now spathose matter has also entered the space 
between the pelvis, coste, and scapule. 
The existence and preservation of the muscular portion of the Crinoidea, has 
been proved by Mr. Parkinson, who placed well preserved portions of columns 
‘in diluted acid, and which gradually removed the calcareous matter, and left 
the fine animal pellicle behind. _ ۱ A 
Since the printing of the description of the Crinoidea, 1 have received Baron 
Von Scnrornuriw's JOrtrefactenkunde, Gotha 1820. In it he gives page 327, 
