MUSEUM OF COMPARATIVE ZOOLOGY. 203 



Plate I. figs. 8 and 9. If the section passed through these flattened 

 joints obliquely, or transverse to their minor axis, a slender leg would be 

 shown ; if through the major axis, a broad swimming leg would be pre- 

 sented. No traces of spines or serrated margins on the inner margins 

 of the basal joints have been observed. 



Good longitudinal sections of the cephalic appendages, in a longi- 

 tudinal section of the Trilobite, have not been obtained. Usually the 

 appendages have the appearance shown in Plate II. figs. 5 and 7, and 

 Plate V. figs. 1, 3, and 4. 



Thus far our description of the cephalic appendages has been derived 

 from sections of Calymene. In Ceraurus, however, we have illus- 

 trations of the same, but in a more fragmentary condition, Plate I. 

 figs. 2, 3, and 4. These and many other sections go to show that the 

 arrangement of the parts is about the same in each species, — the 

 more shallow dorsal shell of Ceraurus necessarily giving a different 

 appearance to the section from one cut at the same place and angle in 

 Calymene. As yet no other appendages have been observed beneath 

 the head that prove to have belonged there. Fragments of the thoracic 

 legs and branchiae are frequently seen in sections crossing the head, 

 but they have been pushed forward and are of accidental occurrence. 



Appendages of the Body. — The appendages of the body or the 

 thoracico-abdominal legs and branchiae are found to vary slightly in 

 the genera Calymene and Ceraurus, as expressed by the species under 

 consideration. The legs of the former are relatively shorter and more 

 symmetrical as compared with the long and somewhat irregularly 

 jointed legs of the latter. 



The Calyniene is frequently found enrolled, the head and pygidium 

 fitting closely together, so that no opening is left at any point, the legs 

 being all drawn within the shell and entirely protected from injury 

 from without. With Ceraurus, i. e. in the species under considera- 

 tion, a perfect closing of the shell by enrolment is impossible, and the 

 space formed by the partial enclosure of the spinous extension of the 

 pleurae affords but an incomplete protection to the numerous legs and 

 branchiae. 



The finest illustration of the legs of Ceraurus, and of the Trilobite 

 as far as yet known, is given in Plate II. figs. 1, 2, and 3. In Fig. 3 

 the form of the transverse section of the basal joint and its mode of 

 attachment to the ventral surface are shown. By Fig. 2 the joints of 

 the leg are shown, and Fig. 1 adds to our knowledge of their shape 



