A MONOGRAPH OF THE EXISTING CRINOIDS 75 



adambulacral deposits in the perisome of the pinnules are much reduced or absent 

 (see Part 2, p. 236). These two families together thus form a distinct group, includ- 

 ing two rather strikingly different types, within the superfamily Mariametrida, this 

 group contrasting rather sharply with the group including the very closely related 

 Himerometridae, Mariametridae, and Colobometridae. 



Comparisons. The members of the superfamily Mariametrida are very easily dis- 

 tinguished from the species of the superfamily Comasterida (see Part 3, p. 76) by the 

 complete absence of a comblike structure on the distal portion of the oral pinnules. 

 In most cases they are also readily distinguished by the central or subcentral mouth; 

 but it must be remembered that in several of the species in the Comasterida the 

 mouth is always, sometimes, or occasionally central. 



The species of Mariametrida are readily distinguished from the species in the super- 

 family Tropiometrida by the middle and distal pinnules, which in the species of 

 Tropiometrida always show a prominent sharp dorsal carination that becomes very 

 conspicuous under a low power when the pinnule is dried. Furthermore, the outer 

 pinnules in the species of Tropiometrida almost invariably are provided with side- 

 plates and covering plates (see Part 2, figures 819-823, p. 392; figures 831-848, p. 405), 

 which are easily visible in dried pinnules under a hand lens. It should be remembered, 

 however, that in the genus Tropiometra these are greatly reduced (see Part 2, fig. 797, 

 p. 372) and are not visible under a hand lens. 



From the species included in the suborder Macrophreata the species of Maria- 

 metrida are distinguished by the relatively robust and short-segmented distal pin- 

 nules in which the first two segments are not appreciably broadened ; by the irregular 

 and relatively wide spacing of the syzygies; by the presence of some trace of carination 

 on at least the basal segments of the oral pinnules; by the more or less abrupt differ- 

 entiation of one or more of the oral pinnules, which are not simply elongated; and by 

 the stronger and less easily detached cirri. 



Most of the species of Mariametrida have more than 10 arms, while in the Macro- 

 phreata very nearly all the included species have 10 arms only, though one has 20 

 owing to the occurrence of 10 instead of 5 radials, and in one or two there may be a 

 few more than 10. The 10-armed species of Mariametrida may be distinguished from 

 the species of Macrophreata by the enlarged proximal pinnules, which are either stout 

 and composed of relatively short segments, or if slender are more or less obviously 

 prismatic with a group of spines, or a conspicuous process, at the distal end of the 

 dorsal ridge on each segment. In the 10-armed species of Mariametrida also the 

 brachials are almost always shorter, and the cirri are stronger and far less likely to be 

 detached from the centrodorsal. The very few species of Macrophreata in which the 

 brachials are short have especially fragile cirri, and P! is composed of a great number 

 of exceedingly short segments, looking more or less like a string of little beads. 



Both the Mariametrida and the Macrophreata include genera containing 5-armed 

 species. In the 5-armed species of Mariametrida (included in the genus Eudiocrinus) 

 the arms are short, the oral pinnules are much enlarged and sharply prismatic, and the 

 second pinnule is on the second ossicle beyond the first (which is on the second post- 

 radial ossicle see fig. 127, p. 79, of Part 2) instead of on the immediately succeeding 

 ossicle as would be expected. In the 5-armed species of Macrophreata (included in the 

 genera Pentametrocrinus and Atopocrinus) the arms are very long with greatly elongated 



