A MONOGRAPH OF THE EXISTING CRINOIDS 5 



converge toward a common type which would fall somewhere within the genus 

 Decametra. 



Thus Petasometra, though quite distinct, is not widely different from Cyllometra. 

 The species of Cyllometra have normally more than 10 arms; but in some of them 10- 

 armed individuals of full size occur which, as Gisl^n has pointed out, would fall in 

 the genus Decametra. 



In most of their characters the smaller species of Colobometra, as C. discolor, 

 approach those species of Decametra, such as D. chadwicki, in which the ends of the 

 pinnule segments and the brachials are more or less spiny. These were formerly 

 placed in a special genus, Prometra, which was assumed to be intermediate between 

 Colobometra and Decametra; but it was found quite impossible to draw any sharp line 

 between Decametra and Prometra, and the latter was suppressed. 



Alisometra and Oligometra are very close to Decametra, although apparently dis- 

 tinct from it. 



The genus darkometra represents a somewhat anomalous type and may for the 

 present be considered as representing a group by itself. It does not seem to me, 

 however, to differ very widely from some of the types included in the genus Decametra. 



The South African genus Embryometra appears to be most nearly related to 

 Clarkometra, though the expansion of the third-fifth segments of the genital pinnules 

 suggests a relationship also with the southeastern Australian Austrometra and the 

 Caribbean Analcidometra. 



The 17 species included within the genus Decametra as now understood form a 

 very heterogeneous assemblage, many of them having little in common with others 

 beyond numerical agreement in details, and hence general proportions. But a sub- 

 division of the genus is impractical on the basis of our present knowledge. 



The family Colobometridae is the largest family of the Mariametrida, including 

 57 species in its 17 genera. It is also the most diversified. The largest species 

 (Pontiometra andersoni) reaches a maximum size only very slightly less than that of 

 the largest species of Ilimerometra, with nearly twice as many arms, while the smallest 

 are smaller than any of the species in the other families. 



It has the greatest geographical range of any of the families of the Mariametrida, 

 being the only family represented in southern Australia, southern Africa, and in the 

 Atlantic (Caribbean Sea); but it has not as yet been reported from the Hawaiian 

 Islands where a species of Lamprometra (Mariametridae) is found. 



Although as in the case of the other families of the Mariametrida the included 

 species are predominantly littoral and sublittoral, the family has a bathymetric 

 range somewhat greater than that of the Mariametridae, and much greater than that 

 of the Himerometridae and Eudiocrinidae. In the Zygometridae one of the genera, 

 Zygometra, has a bathymetrical range that is approximately the same as that of the 

 Himerometridae, and therefore much less than that of the Colobometridae; but the 

 other, Catoptometra, has a bathymetrical range of nearly 900 meters, greater than 

 the range of all the other genera in the Mariametrida together. 



Hiniory. — In the Challenger report on the comatulids published in 1888 Dr. P. H. 

 Carpenter placed the 10-armed species known to him that are now assigned to the 

 family Colobometridae in the Milberti group {Antedon [Oligometra] serripinna, A. 



