786 " THETIS " SCIENTIFIC RESULTS. 



less certainly to either Plilometra mulleri or P. macronenia, bub 

 which of the two it is impossible to say with accuracy. 



The stage represented is somewhat in advance of that described 

 as " poidophora," more nearly coinciding with " tvUsoni." 



Tlie ten arms are 25 mm. long, and the cirri, which are xi, two 

 to each radial area excepting one which has three, are 28 mm. to 

 33 mm. long; the first cirrus segment is short, the second nearly 

 or quite as long as broad, the third slightly longer, the fourth 

 nearly or quite half again as long as broad ; the following five or 

 six segments are similar, the succeeding then gradually decreasing 

 in length so that the outer segments are about as long as broad ; 

 these shorter distal segments are bluntly carinate ; one of the 

 cirri has the tip broken and regenerating as often seen in this 

 species. 



The elements of the I Br series have already taken on the 

 characters found in the adult; the I Br^ is oblong, two and one- 

 half times as broad as long, the I Brg broadly pentagonal with the 

 lateral edges about half tlie length of those of the I Br^ ; the 

 elements of the I Br series and the first two brachials have the 

 lateral edges perfectly straight and somewhat produced into a 

 narrow flange-like border by which they are in lateral apposition ; 

 a broad synarthrial tubercle is just beginning to develop. 



The arms are rounded dorsally with as yet no trace of carina- 

 tion ; the pinnules are rounded and not prismatic, though the 

 larger show traces of a prismatic condition in the basal segments ; 

 the marginal lappets of the pinnule ambulacra are calcified, but 

 side plates have not yet become differentiated. Syzygies occur 

 between the third and fourth and seventh and eighth brachials, 

 and distally at intervals of two oblique muscular articulations. 



While at Cambridge, Massachussetts, in the summer of 1908, 

 I was enabled, thanks to the kindness of Dr. H. L. Clark, to 

 •examine some very young specimens, three in number, of 

 Ptilometra mulleri ; these present many points of difference from 

 the adults, and may be described as follows : — 



Centrodorsal thick-discoidal, bearing vii-ix cirri in a single 

 Toarginal row ; these have 25-30 segments of which the two first 

 are about twice as broad as long, the third somewhat longer, the 

 fourth and following about as long as broad ; the segments in 

 the outer half of the cirri have the i)roximal edge of the dorsal 

 half cut away, and are somewhat compressed dorsally, being 

 almost carinate on some of the cirri; penultimate segment with 

 a prominent opposing spine which is terminally situated and 

 reaches in height about one-half the lateral diameter of the 

 segment bearing it ; the cirri are comparatively thick at the base 

 and decrease gradually in diameter until about the fourth segment 

 after which they remain of the same diameter. 



