PART 5 A MONOGRAPH OF THE EXISTING CRDSTOIDS 47 



50 mm. long, with about half as many species ranging up to SO 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 proximal 

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

 become almost or quite triangular, usually about as long as broad, and later wedge- 

 shaped again, gradually elongating distal!}- 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 all 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 hi these last the arm 

 division, though occurring 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 fall into two distinct 

 classes, (a) with P 3 as well as P 2 and PI modified and differing from the succeeding 

 pinnules, and (b) with P 3 of the same size and character as the succeeding pinnules. 



In the first group, PI, P 2 and P 3 may be similar and about the same length, longer 

 than those succeeding (Iridometra) ; P 2 may be the largest and longest, though with P 3 

 of the same character and not much smaller (Andrometra) ; or P 3 may be the longest 

 and stoutest pinnule on the arm, with PI and P 2 smaller and similar to each other, or 

 with P 2 intermediate in size between PI and P 3 (Toxometra, Dorometra and Eumetra). 



In the second group two genera (Argyromctra and Euantedon) have PI and P 2 with 

 rarely more than 21 segments, P 2 being smaller except in Argyrometra mortenseni. In 

 one genus (Annametra) P : and P 2 are similar, the latter being slightly longer than the 

 former, with 18 to 35 segments. 



In the remaining genera (Mastigometra and Antedon) P t is greatly elongated and 

 much the longest pinnule, and P 2 may be intermediate between it and P 3 , or of the 

 same length as P 3 but ungrooved and without a gonad, or quite similar to P 3 in all wnys. 



The segments of ah 1 the oral pinnules 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 ambulacral grooves of the pinnules consist 

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



