MUSEUM OF COMPARATIVE ZOOLOGY. 217 



DESCRIPTION OF THE SECTIONS, ETC. USED FOR 

 ILLUSTRATION. 



PLATE I. 



Fig. 1. Transverse section, perpendicular to the median axis, of the head of 

 Ceraurus pleurexantJiemus, on a line intersecting the eyes. The 

 central lobe of the cephalic shield and hj'postoma (/i) entirely 

 enclose the median cephalic cavity, which, in its present state, is 

 filled with calcspar. 



Fig. 2. A transverse section, cutting obliquely, downward from near the pos- 

 terior portion of the glabellar lobe to a point beneath, and a little 

 back from the eyes. A ditsinct space occurs between the dorsal 

 shell and hypostoma which is occupied by fragments of the mandu- 

 catory appendages. 



Fig. 3. This section is taken a little posterior to the preceding, and terminates 

 beneath, farther back. The cephalic appendages are shown only 

 near their bases. 



Fig. 4. This is one of the most satisfactory sections obtained of the cephalic 

 region of this species. In this and the three preceding, the ap- 

 pendages have been, apparently, crushed back between the hypostoma 

 and dorsal shell, so as to give an incorrect idea of their true position 

 when seen in the sections. In Fig. 4 the jointed appendages are 

 indicated, and also the manner of their arrangement, the leg on the 

 left side showing three joints. This would hardly be intelligible 

 were it not for the sections of the species next to be mentioned. The 

 four sections taken to illustrate the head of Ceraurus are among 

 many that show similar features in about the same condition of 

 preservation. The branchiae shown in Figs. 3 and 4 are not consid- 

 ered cephalic, but as thoracic, and pushed beneath the cephalic 

 shield after the death of the animal. 



Fig. 5. A section of the head not far from the position of that illustrated by 

 Fig. 4, but by reason of the enrolment of the animal, portions of the 

 thoracic appendages are cut across, and, as mentioned of Figs. 1-4, 

 the latter have also been forced beneath the head. One of the most 

 interesting features of this section is the ribbon-like branchia;, h, h. 

 The appendage o is probably cephalic ; all others, thoracic. 



Fig. 6. With this section we commence a series of five cut from different in- 

 dividuals of the species Cahjmene senaria. All show the cephalic 

 appendages. Without exception they are cut transversely and 

 obliquely dovni through the head from just back of the posterior 

 segment to the lower anterior side, intersecting the hypostoma (h) 

 towards its posterior end. The posterior or fourth pair of appen- 



