MONOGRAPH OF THE EXISTING CRINOIDS. 151 



out the group than are the oral pinnules, though much less uniform than the distal 

 pinnules. The first two segments are short, the first trapezoidal with the longest 

 side outward and about twice as broad as long, the second trapezoidal with the base 

 inward, joined to the longest side of the first, and about of the same length. The 

 first segment is often more or less enlarged in the earlier genital pinnules, indicating 

 an abrupt decrease in the size of the pinnule as a whole which has not yet affected 

 the basal segments to the same extent that it has affected the others. Farther out 

 the second segment gradually becomes longer, about twice as long as broad at the 

 base, as in the distal pinnules. The third segment as a rule is squarish, the follow- 

 ing very gradually becoming longer and elongating rather rapidly in the distal por- 

 tion. In the Macrophreata the relative length of the component segments is much 

 greater than it is in the Oligophreata. 



In the Macrophreata there is very rarely any special differentiation of the 

 segments of the genital pinnules for the protection of the gonads : they are merely 

 slightly broader than those of the distal pinnules; but in the genera Notocrinus and 

 Isometra (fig. 1239, pi. 38), both from the antarctic, the third-sixth segments 

 are much broadened. 



In the Oligophreata the segments of the genital pinnules are almost always 

 slightly broadened from the third outward, though usually not so much so as to 

 alter their general symmetry. This is well seen in the Comasteridse (figs. 262, 

 265, p. 207) and in the Thalassometriclse (figs. 332, p. 227, and 336-338. p. 229). In 

 a few groups there is a very marked modification of the genital pinnules. In the 

 genus Comatula (figs. 184, p. 100, and 262, p. 207) the third and following segments 

 are very short, much broader than long, though the pinnules as a whole are not 

 especially widened. In the Charitometridso (figs. 302-306, p. 223, and 1055, 1057. 

 1058, 1061, pi. 14), to a much less extent in the Calometridse, and in one genus each 

 in the Thalassometridae (Hone&mefra [fig. 321. p. 227] ) and Colobometridae (Aus- 

 trometm [figs. 340, 341, p. 229]), the third segment is abruptly widened and the 

 fourth even more so ; this widening may continue for one or two, or even half a 

 dozen more segments, and then cease abruptly, or it may gradually taper away 

 distally, disappearing toward the tip of the pinnule. The first condition is well 

 illustrated by Pcecilometra (figs. 1057, 1058. pi. 14), Horceome-tra (fig. 321, p. 227). 

 and Strotom-etra (figs. 230-232. pp. 118-192), and the second by Crinometra (figs. 

 302-306, p. 223), Crossometra, Perissometra (fig. 1055, pi. 14), Pachylometra, Glyp- 

 tometra, C hlorometra, C'hondrometra, the genera of the family Calometridse, and 

 Austromet-ra (figs. 340, 341, p. 229). It is worthy of note that abrupt expansion of 

 the segments of the genital pinnules is confined to 10-armed species. The broaden- 

 ing of the pinnulars results in the formation of a roof over the entire dorsal surface 

 of the gonads, which protects them from injury. 



P. H. Carpenter first noticed that the grooveless condition, which is typically 

 and usually limited to the oral pinnules, sometimes exists on whole arms and on 

 all the pinnules borne by them in many species of Comasteridse (part 1, fig. 45, i, 

 p. 79). Even in the arms which arise from the anterior or oral side of the disk the 

 ambulacral groove does not give off regular branches to the pinnules borne bv the 

 142140 21 Bull. 82 12 



