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MUSEUM OF COMPARATIVE ZOOLOGY. 199 
description of the material used, as the basis of the conclusions given 
in this and the succeeding chapter, than will be obtained from the 
present general descriptions. 
The Ventral Membrane.— In those longitudinal sections in which 
the ventral membrane is most perfectly preserved, it is shown to have 
been a thin, delicate pellicle or membrane, strengthened in each segment 
by a transverse arch, to which the appendages were attached. These 
arches appear as flat bands separated by a thin connecting membrane, 
somewhat as the arches in the ventral surface of some of the Macrouran 
Decapods. The finest illustrations of this structure have been found 
in Calymene, but several sections of Ceraurus show it very well 
defined. The section represented in Plate V. fig. 2, gives a very fine 
view of the membrane and arches in a longitudinal section. These 
parts are also shown by the section crossing the Trilobite diagonally to 
the median lobe (Plate V. fig. 4), and also the variation of the form of 
the arch near the point of the attachment of the leg. This point is 
seen in Plate V. figs. 1, 2, and 3. 
In by far the greater number of sections, both transverse and longi- 
tudinal, the evidence of the former presence of an exterior membrane, 
protecting the contents of the visceral cavity, rests on the fact that the 
sections show a definite boundary line between the white calespar, 
filling the space formerly occupied by the viscera, and the dark lime- 
stone matrix. Even the thickened arches are rarely seen. This is 
almost universally the case with the legs and attached appendages, as 
their external membrane is not to be distinguished as such. It would 
appear that in the process of mineralization the calespar that replaced 
the viscera and contents of the appendages also replaced the substance 
of the membrane, thus forming one continuous mass and effacing all 
traces of the delicate external test. The nature of this covering is also 
shown by the present imperfect condition of the appendages. Only in 
a few rare instances are they found in an approximately perfect state, 
and the many bizarre forms prove that it was semi-elastic, often under- 
going maceration, and thus forced into many irregular forms, as shown 
in Plate II. figs. 6 and 8, and Plate III. figs. 3, 4, and 5. 
On the same small block of limestone with the two jointed legs illus- 
trated on Plate VI. fig. 5, occur the remains of the dorsal shell of both 
Calymene senaria and Trinucleus concentricus. The contrast in the 
test of the joints forming the legs and that of the dorsal shell is very 
striking. The latter is firm, thick, and of a yellow or opalescent color, 
while the former is of a bronze color, thin, indented with numerous 
