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 Memhrane. — 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 Calyniene, 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. tig. 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 calcspar, 

 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 calcspar 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 Trimicleus coJicentricus. 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 



