218 BULLETIN OF THE 



dages is well shown in all, 4, 4, of the figures, and the first three 

 pairs the most distinctly in Figs. 9 and 10 (1, 2, 3, Fig. 10), although 

 portions are finely illustrated in Figs. 6 and 8. Figs. 8 and 9 are 

 considered as denoting the presence of swimming joints (n, n). 

 The organs b, b of Fig. 8 are thoracic, comparable to b, b of Fig. 5. 



In all the sections illustrated on this plate there is no connection 

 of the central portion or visceral cavity with the doublure or 

 incurved portion of the margin of the dorsal shell. The union of 

 the portions of the membrane and enclosed organic substance, re- 

 placed by calcspar, in Figs. 6 and 8 a, a, and 4, would make such 

 a connection complete. In many other sections not illustrated the 

 same conditions prevail, so that we know that the space between 

 the central mass of appendages and the margin of the dorsal shell 

 was united by a thin membrane that left a narrow space between it 

 and the dorsal shell. The suggestion made by Mr. E. Billings, 

 that the central mass of legs, etc. of the Trilobite was probably 

 supported in the same manner as the same parts in Limulus, is thus 

 sho'v\ni to be correct. 



PLATE II. 



Fig. 1. A transverse section of the thorax of Geraurus. As is the case with 

 the sections of the head, the central mass of the body is not united 

 to the margins of the pleurae in any of the sections, but the proof of 

 such a union in the same manner as in the sections of the head is 

 found in portions of such a membrane existing iu many sections. 



This section is of an enrolled individual. As the outline of the 

 cephalic shield and central portion of the cephalic cavity only are 

 shown in the lower portion, the half cutting across the thorax is used 

 for illustration. The position and character of the jointed ambu- 

 latory legs is beautifully shown, and also the presence of the bran- 

 chiae : the left leg, as in the figure, gives the basal joint and an 

 outline of the succeeding joints ; this is still more finely shown by 

 the leg on the right side. The space occupied by the visceral cavity 

 is compressed and filled with calcspar. No remains of the intes- 

 tinal canal are to be distinguished. 



Fig. 2. A section of the right side of the thorax of another enrolled individual. 

 The spar filling the hypostoma is seen at h. The Trilobite was 

 broken and the left side lost, so that but one half of the transverse 

 section could be obtained. The leg is broken near its base, and also 

 towards the extremity. It appears to be formed of seven joints, 

 and perhaps eight, if the terminal portion is not a fragment of 

 another leg brought into the plane ol the section in line with the 

 other joints. The branchial appendages were misplaced by the 

 movement that carried the leg to the left, as seen in the figure. 



