A MONOGRAPH OF THE EXISTING CRINOIDS 3 



Remarks. Though highly diversified, the cirri of the numerous species in this 

 family always, either throughout or on a greater or lesser proportion of their seg- 

 ments, have dorsal processes consisting of a serrate or tubercular ridge across the 

 segment, or a pair of spines. Sometimes there is a transverse ridge on the basal seg- 

 ments which soon resolves itself into a pair of spines, or the earlier segments may 

 bear a transverse ridge or a pair of spines that soon -sometimes almost immedi- 

 ately gives place to a single conspicuous dorsal spine, as in the case of Pontiometra 

 and Cotylometra. In these two genera most of the cirrus segments have single dorsal 

 spines, and unless the basal segments are examined carefully the characteristic fea- 

 ture of the cirri will be overlooked. In Epimetra most of the cirri have a fine dor- 

 sal carination only; but here the broadly bilobate or forked opposing spine attracts 

 attention at once. 



In the other families of the Mariametrida the cirrus segments are dorsally 

 smooth, sharply carinate, or bear more or less strongly developed dorsal spines with 

 their elongated bases lying along the middorsal line. Only in Amphimetra tessellata 

 (see part 4a, p. 360) do we occasionally, though very rarely, find some of the earliest 

 dorsal spines paired. This would seem to suggest that the somewhat anomalous genus 

 Amphimetra has certain affinities with this family. Its very short brachials ally it 

 more closely, however, with the Himerometridae, while in the very rare instances in 

 which A. tessellata has more than 10 arms the IIBr series are always 2, as in the 

 Mariametridae and in most of the multibracbiate species in this family. 



The 17 genera included in the family Colobometridae appear at first sight to 

 form a very heterogeneous assemblage, contrasting rather sharply with the homo- 

 geneity so characteristic of the families Himerometridae and Mariametridae. Yet 

 when we examine them closely we can see, in spite of the apparent wide dissimilarity 

 of the extremes, a number of features by which they are linked together, and at the 

 same time separated from the genera in neighboring families. 



These 17 genera may be segregated into five generic groups. The first group 

 includes the genera Pontiometra, Basilometra, and Epimetra, which agree in having 40 

 or more arms with extraordinarily narrow division series, stout cirri with at least 40 

 segments of which the longest are only slightly longer than broad, and one or more of 

 the proximal pinnules greatly elongated. But here the resemblance ends. The three 

 genera differ widely in practically every other feature. In Basilometra the IIBr and 

 IVBr and following division series are 4(3+4), and the IIIBr series are 2. In 

 Pontiometra all the division series are normally 2, but division series of 4(3+4) occur 

 exceptionally. In Epimetra all the division series are 2. Pontiometra, with its 

 smooth, enormously elongated and stiffened though not rigid PI, is unique. The rigid 

 proximal pinnules of Basilometra and Epimetra, composed of much elongated segments 

 with spinous ends, are structurally the same as the proximal pinnules of the species of 

 Colobometra, some of which also possess cirri with a large number of segments. And 

 we must not overlook the fact that the relation of the long and stiff Pj of Pontiometra 

 to the rigid proximal pinnules of Basilometra and Epimetra is essentially the same as 

 the relation between the long and stiffened Pa in Stephanometra indica and the rigid 

 and spinelike P 2 in the very closely related perhaps in reality identical S. protectus. 

 In Pontiometra all the proximal pinnules are present. In Basilometra and in Epimetra 

 P a is always absent, while PI and often P 2 and P b are also absent from the inner arms 



