PART 5 A MONOGRAPH OF THE EXISTING CRINOIDS 47 



50 mm. long, with about half as many species ranging up to 80 mm., and about half 

 this number above 80 mm. 



In structure the arms show a remarkable uniformity; between the first and second 

 syzygies and somewhat beyond, the brachials are wedge-shaped, with the pro.xiraal 

 and distal edges concave, usually about twice as broad as the median length; they then 

 become ahnost or quite triangular, usually about as long as broad, and later wed^e- 

 shaped again, gradually elongating distally where the articulations are usually more 

 or less swollen. The distal edges of the brachials are usually finely and inconspicuously 

 spinous, very rarely prominently so. The dorsal surface is never carinate or otherwise 

 modified. 



Syzygies occur almost invariably between brachials 3+4, 9 + 10, and 14 + 15, 

 and distally at intervals of 3 muscular articulations. 



In only three species, all belonging to the genus Antedon, has a tendency to an 

 increase in the number of arms over 10 been observed. Antedon bifida has been found 

 with 11 and with 12 arms, the IIBr series having been 2 in aU cases but one in which 

 it was 4(3+4); A. bifida moroccana has been reported with 11 arms, one IIBr 

 series of 2 being present; A. mediterranea and A. petasus with 12 arms have also been 

 recorded, besides three specimens of petasus with 11 arms, but in these last the arm 

 division, though occiuring at the base, was abnormal in character. Possibly when other 

 genera have been collected as widely as has Antedon similar anomalies will be found. 



In regard to the oral pinnules, the genera of Antedoninae faU into two distinct 

 classes, (a) with P3 as well as P2 and Pi modified and differing from the succeeding 

 pinnules, and (b) with P3 of the same size and character as the succeeding pinmdes. 



In the first group, P,, Pj and P3 may be similar and about the same length, longer 

 than those succeeding (Iridometra) ; Po may be the largest and longest, though with P3 

 of the same character and not much smaller {Andrometra) ; or P3 may be the longest 

 and stoutest pinnule on the arm, mth Pi and P2 smaller and similar to each other, or 

 with P2 intermediate in size between P, and P3 {Toxometra, Dorometra and Eumetra). 



In the second gi-oup two genera {Argyrometra and Euantedon) have Pj and P2 with 

 rarely more than 21 segments, P2 being smaller except in Argyrometra mortenseni. In 

 one genus {Annmnetra) Pj and P2 are sLnailar, the latter being slightly longer than the 

 former, with IS to 35 segments. 



In the remaining genera (Mastigometra and Antedon) P, is gi'eatly elongated and 

 much the longest pinnule, and P2 may be intermediate between it and P3, or of the 

 same length as P3 but migrooved and without a gonad, or quite similar to P3 in all ways. 



The segments of all the oral pinmdes beyond the basal are elongate and more 

 than twice as long as broad in all the genera but Annametra, though they are never ex- 

 cessively long. In Antedon and in Mastigometra they are less elongated than in the 

 remaining genera. When long the oral pinnules always become stiffened, at least in 

 the basal half or two-thirds. 



The middle and distal pinnules show very little variation throughout the group; 

 the distal edges of the component segments, at least the outer ones, are always more or 

 less armed with spines, and this spiny border may become highly developed in some 

 species of Antedon. 



The deposits along the sides of the ambulacra! grooves of the pinnules consist 

 usually of a single slender and smooth calcareous rod ui each lappet which is straight 



