MONOGRAPH OF THE EXISTING CRINOIDS. 131 



Heterometra phittberti, which has very numerous arms with very short brachials 

 and all the division series except the first 4(3+4), and II. crenulata, which also has 

 very numerous arms composed of very short brachials, possibly approach more or 

 less closely the type originally characteristic of the genus. But most of the species 

 of Heterometra, like those of Craspedometra (part 1, tig. 85, p. 139), Himerometra 

 and Homalometra, fall within the broad generalization. 



Thysanometra tenelloides, though having only 10 arms, has the brachials very 

 short and wedge-shaped proximally, becoming narrowly oblong in dorsal view 

 distally. The syzygial pairs, however, are distributed as in normal 10-armed types. 



In the very large species of the Heliometrinse and in broad-armed species of 

 the Comasteridse (figs. 242, p. 197, and 256. p. 205) the brachials show a tendency to 

 shorten and to approach the type of brachial typically found in species with IIBr 

 series. 



In arms arising from a IIBr or later division series the syzygial pairs are less 

 frequent than in arms arising from the IBr series; the first syzygial pair, between 

 the third and fourth brachials, is always present, the second is usually absent, the 

 third is more or less variable in position, and the distal are separated by from four 

 to six or seven or more brachials; often, indeed, they maj' be separated by long 

 intervals, and sometimes they may be almost completely lacking. Thus in a speci- 

 men of Cenometra ~bella at hand, while the first syzygial pair is in all cases present, 

 there may be no others in the arm. If there be a second it is composed of the 

 forty-second and forty-third brachials (twice), forty-third and forty-fourth, forty- 

 fourth and forty-fifth, forty-eighth and forty-ninth, fifty-fifth and fifty-sixth, or 

 ninety-first and ninety-second. 



In this connection it is interesting to note that in the Pentametrocrinidae and in 

 Atelecrinus the distal intersyzygial interval is greater than that usual for 10-armed 

 species, and also less regular, being from one to six brachials, usually from 

 two to five. 



ARM TIPS. 



The comatulid arm ordinarily ends in a growing tip which is slowly adding 

 new brachials, together with the accompanying pinnules (fig. 288, p. 221). The 

 terminal 12 or 15 pinnules, therefore, are progressively less and less mature and 

 decrease gradually in length, the outermost being represented merely by a bud. 

 In the adults the rate of growth of the arm is exceedingly slow, and the same 

 is true of the terminal pinnules, which retain their relative proportions after all 

 growth has practically ceased. 



In the species of Calometridae and Ptilometrina? (part 1, fig. 46, p. SI), 

 especially well illustrated in the latter, the arm ends abruptly with half a dozen 

 very short crescentic brachials, this abortive tip being recurved and more or less 

 concealed between the outer pinnules. The terminal pinnules, like the terminal 

 brachials, decrease very rapidly in length, so that the subterminal pinnules exceed 

 them and when the pinnules are closed together reach several millimeters beyond 

 the arm tip giving the arm the appearance of having been broken off. A greater 

 or lesser approach to this condition is seen in the Tropiometridse (fig. 291. p. 221) 



