OPIIIOCOMA ALEXANDRI. 



75 



arms cross-barred with darker l^ands, from two to four joints on each 

 band ; some of the upper arm-plates finely marliled with lighter ; below, 

 mouth parts and interbrachial spaces having several shades of brownish 

 yellow ; under surface of arms light-brown, a longitudinal l)rown line 

 runs along the lateral sides of the under arm-plates. 



Variations. — The pattern of the color varies little, but the ground 

 tints may have more of yellowish, gray, or brown ; it very rarely hap- 

 pens that the arms are not banded. The proportions of arms to disk 

 give such differences as the following : 12 : 65, 15 : 100, 10 : 135, 

 17.5 : 11-. The largest specimen 1 have seen had a disk of 22°""", 

 A^ery large specimens have seven spines on some of the joints, and 

 six on most ; where there are six spines, it is the fourth that is longest. 

 A young one, with a disk of 3.2"""-, had arms of about 22'"'"- ; the disk 

 was covered with imbricated scales, with a few large grains ; the upper 

 and under arm-plates were similar to those of the adult, but longer ; 

 the arm-spines were four, rounded, tapering, and much stouter than 

 those of the adult ; the number of mouth-papilla3 was the same as in 

 older ones, but there Avere only two tooth-papilla?. I have seen no 

 Aoung ones with six arms. 



O. Alexandri was first received from my friend, Mr. Alex. Agassiz, 

 after whom I have named it. It resembles 0. piimila, but differs in 

 having the third or fourth arm-spine much longer than the rest. 



LIST OF SPECIMENS. 



