MONOGRAPH OF THE EXISTING CRINOIDS. 101 



are well separated; but in Perometra (part 1, fig. 112, p. 179), Balanometra, Zeno- 

 metra, certain species of Trichometra, Nanometra, and related types, and in certain 

 specimens of the species of Leptometra, the postradial series are in close apposition 

 and more or less, sometimes very greatly, flattened, this lateral flattening reaching 

 in Zenometra columnaris and in Trichometra cubensis almost the same extreme as 

 in the most characteristically " wall-sided " species of the Thalassometridse. In 

 Atelecrinus (part 1, figs. 123, p. 192, 124, 125, p. 193, and 414, p. 319) and in 

 Hypalometra the constriction of the lower part of the animal is so great that, 

 though no great flattening occurs, thanks to the very considerable depth of the 

 brachials, the lower pinnules have no room in which to develop, and are there- 

 fore absent. 



An instructive side light on the uncertainty of the " wall-sidedness " as a spe- 

 cific character is given by an examination of the " Basicurva group " as constituted 

 by Carpenter in the Challenger report. All of the included species but two fall in 

 the families Thalassometridse and Charitometridse, as now understood. Of these 

 two, one belongs to the genus Perometra (pusilla) of the family Antedonidse and 

 the subfamily Perometrinae, and the other to the genus Homalometra (denticulata) 

 of the family Himerometridse, both being much more closely related to species 

 placed by Carpenter in his heterogeneous " Milberti group " than to any of the 

 species with which they were associated. 



In the groups in which the lower portions of the poftradial series are flattened 

 against each other and thus definitely fixed in position the general contour of the 

 lower part of the animal comes to present valid systematic characters which are 

 unsurpassed in reliability by any of the characters offered by any of the other 

 structures. This is especially true in Crossometra (figs. 226, 673, pp. 180, 335), 

 Perissometra (figs. 227, 670, pp. 182, 335, and part 1. fig. 100, p. 162) and Pachylo- 

 metra (figs. 671, 672, p. 335) in the Charitometridse, and in Ptcrometra (fig. 203, p. 

 137) and Aslerometra (figs. 206, 207, 209, pp. 143-149, and part 1, fig. 94, p. 155) in 

 the Thalassometridse. It was first used as a differential character by Carpenter in 

 distinguishing Perissometra angusticalyx, in which, as in certain more or less 

 related species, and in certain species of Pterometra, that part of the animal just 

 above the radials is strongly constricted. 



When the lower portions of the postradial series are not flattened against each 

 other the contour of the proximal portion of the body depends mainly, in preserved 

 specimens, upon the state of contraction and distortion of the visceral mass, and 

 has either no, or only a very general, significance. 



The close apposition and the strong flattening of the earlier postradial elements 

 against each other would result, were it not for some special adaptation, in the 

 cutting off of the medium inhabited by the animals from contact with the surface of 

 the visceral mass, entirely from its sides and more or less completely from its 

 ventral surface. This would result in the partial stagnation of the fluid whence 

 the contents of the water vascular system are drawn, and would undoubtedly give 

 rise, directly or indirectly, to serious pathological conditions. But in most cases 

 obvious provision has been made against such a contingency. The IB^ and all 



