398 bulletin' 82, UNITED STATES NATIONAL MUSEUM 



height equal to the distal width of that segment. The terminal claw is nearly as long 

 as the two preceding segments, slender and only slightly curved. 



The disk and the brachial ambulacra are naked. Side and covering plates are 

 well developed in the distal half of the pinnule ambulacra. 



The ends of the basal rays are visible as rhombic tubercles in the interradial 

 angles of the calyx, but do not project above the general surface of the radials. 



The radials are rather promment, nearly four times as broad as long, gently 

 concave distally. The IBrj are about two and one-half times as broad as long, with 

 their straight lateral edges in apposition with those of their neighbors. The IBrj 

 (axillaries) are rhombic, about twice as broad as long, the lateral edges about half as 

 long as those of the IBrj. The IIBr series are 2, developed on the right side of 3 of 

 the IBr series, on both sides of one, and absent from the fifth. The division series 

 and first 2 brachials are broad and outwardly have a uniformly produced narrow 

 border by which they are in lateral apposition, though they are not laterally flattened. 



The 1 5 arms are about 55 mm. long. The brachials in general resemble those of 

 P. australis, but in the distal half of the arm they are sharply rounded dorsally in- 

 stead of sharply carinate, the long overlapping spines of that species being represented 

 by insignificant tubercles on the median portion of the distal edge of each segment. 



Syzygies occur between brachials 3 + 4, again between brachials 12 + 13 or 13 + 14, 

 and distally at intervals of 5-7 (usually 6 or 7) muscular articulations. 



Pi is 8 mm. long, stiff, rather small, with 9 segments of which the first is not quite 

 so long as broad, the second is slightly longer than broad, the third is twice as long 

 as its proximal width, and the remainder are greatly elongated, except for the minute 

 terminal segment. Pj is 12 mm. long, much stouter than Pi, with 9 segments which 

 are similar in their proportions to those of Pi. P3 is the longest pinnule on the arm, 

 13 mm. or 14 mm. in length, with 11 segments which are of similar proportions to 

 those of the preceding pinnules. P4 is intermediate m length between Pi and P2 11 

 mm. long, with 11-12 segments of which the first is twice as broad as long, the second 

 is about as long as broad, and the following gradually increase in length, becoming 

 greatly elongated distally. The pinnules succeeding are similar to about the middle 

 of the arm, beyond which point they become more slender, slightly less stiff, and 

 decrease in length distally to about 9 mm. P] is occasionally absent, in which event 

 P2 is much smaller than usual. 



The color in alcohol is dull purple. 



Notes. — The preceding description is based upon the specimen from Dirk Hartog 

 Island, the type of Ptilometra dorcadis. 



The largest specimen presumably from the vicinity of Perth has 19 arms. The 

 centrodorsal is thick -discoidal or columnar with the dorsal pole very slightly concave, 

 6 nam. in diameter. The cirri are XXXV, 89-106, the longest 65 mm. long. They 

 are stout basally but taper in the proximal third and become slender in the distal 

 half. The first segment is short and those succeeding gradually increase in length 

 becoming nearly or quite as long as broad on the tenth or eleventh, remaining of the 

 same proportions to nearly the end of the proximal half, then gradually decreasing in 

 length so that the segments in the distal third are about twice as broad as long. With 

 the shortening of the segments distally the distal dorsal edge begins to become pro- 

 duced, this production on the short distal segments forming a high narrow carination 



