296 BULLETIN 93, UNITED STATES NATIONAL MUSEUM. 



a'. Spines between the fourth tooth and lower point are fewer, compara- 

 tively coarse, and distinct, easily counted (fig. 90) C. malayensis. 



II. Mandible having three large teeth and a broad lower extremity set with 

 many spines, which are largest at the end, decreasing above and be- 

 low it. 

 l . Without caudal appendages. 



b 1 . Anterior ruinus of cirrus iii much longer than posterior, with spines 



in circles on the distal segments (7. antcnnatits. 1 



b 2 . Third pair of cirri much longer than second, second pair having coarsely 



pectinated terminal spines C. dental us* 



b 3 . Third pair of cirri having a few basal segments thickly covered with 



spines, like the second pair C. intcrtcxtus. 1 



I*. Maxilla and cirri normal C. witlicrsi (fig. 91). 



b 5 . Maxilla densely crowded with subequal spines. Posterior ramus of 

 cirrus ii long, like the following cirri ; dense short tufts between the 



spines of the later cirri C. hembeli (fig. 97). 



a 3 . Having long caudal appendages- C. caurlatus (fig. 92). 



KEY TO SPECIES OF CHTHAJ1ALUS. 



The keys for the determination of specimens constructed by Hoek 

 and Gruvel from Darwin's descriptions will not work in actual prac- 

 tice. Gruvel's key is workable with the most perfect specimens, but 

 not with much of the material one encounters, since the characters 

 used are often corroded away. The following key is highly artificial 

 and not very satisfactory : 



a\ Very large, solid and strong, with zigzag sutures ; base calcareous in adults ; 



tergal margin of the scutum longer than the basal C. licmbcli (p. 324). 



a 2 . Smaller, diameter usually 5 to 15 mm. ; not so strong. 



b l . Right and left opercular valves very unlike in size and shape. 



C. anisopoma (p. 317). 



b-. Opercular valves nearly or quite alike on the two sides in normal specimens. 

 c 1 . Interior of a uniform very dark (purplish) color. 



d 1 . Interior rich violet, the basal margin inflexed ; sutures, when unworn, 

 of interfolded laminae. Tergum very narrow in the lower half, 



calcified to the scutum C. intcrtc.rtus (p. 324). 



d'\ Interior blackish violet, without basal ledge ; sutures simple. Tergum 

 narrow, with a short, rounded spur, not calcified to the scutum, 

 which has a very low articular ridge and no pits for the muscle- 

 insertions C. imperatrix (p. 320). 



c 2 . Interior not uniformly very dark. 

 d 1 . Adductor ridge of scutum distinct. 



c 1 . Tergum broad in the lower part, its basal margin not distinctly 



siuuated below the depressor crests. 

 f. Tergum triangular, equilateral; tergal margin of the scutum 



short, its adductor ridge small C. fissus (p. 317). 



f. Tergum longer than wide; tergal margin of scutum longer, the 

 adductor ridge long and strong C. dalli (p. 316). 



1 1 have not dissected C. scabrosits., C. antennatits, C. dcntatus, or C. interteorttts, in- 

 cluded in the above table, the details being taken from Darwin. As none of the mandibles 

 have been figured, it is possible that some of these species are incorrectly grouped. 



