MONOGRAPH OF THE EXISTING CRINOIDS. 147 



metra the proximal pinnules are comparable to those of such species of Stephano- 

 metra as S. echinus or S. tenuipinna, though they are proportionately somewhat 

 longer and stouter. In OUyometra scrripinna (fig. 315, p. 227) and in 0. carpen- 

 teri the oral pinnules are strictly comparable to those of Heterometra crenulata, 

 and the segments of the second or second and third pairs have the same broad 

 processes on their prismatic ridges. In Oligonietra japonica and in 0. caledonice 

 the lower pinnules are comparable to those of Amphimetra spectabilia or A. dis- 

 coidea (fig. 309, p. 223). Cennmet7-a (figs. 274, 278, p. 213) has essentially the same 

 proximal pinnule structure as Stephanoiiwtr'o, monacantha (fig. 310, p. 223), but 

 Pj is usually much stouter and more as in Himeronietra. In the species of the 

 genus Colohometra we find a close parallel to the species of the genus Stephano- 

 metra (see figs. 273, p. 207. and 325, p. 227). Most of the species have P^ delicate, 

 many jointed and flexible, and from one to six of the following pinnules on either 

 side greatly enlarged, stiffened and spine-like, composed of segments with ex- 

 ceedingly spinous distal ends. Except for this last feature, characteristic of 

 the Colobometridse, these species are comparable to the large species of Stephano- 

 metra, while those with fewer enlarged pinnules are comparable to S. spicata, 

 S. monacantha, or S. indica. One species, C. diadema (fig. 325, p. 227), has Pi 

 of the same character as P,, thus being similar to Stephanotnetra tenuipinna or to 

 S. echinus. But all the species of the genus Colohometra are peculiar in lacldng 

 Pa, though this pinnule commonly reappears on regenerated arms. The oral 

 pinnules of Cyllometra, Decametra. and Provietra may best be compared with 

 those of the species of Lamprovietra (fig. 312, p. 223) or Dichrometra (fig. 313, p. 

 223) from which they differ only in having a greater or lesser development of spines 

 on the distal borders of their component segments. In some forms, as in Deca- ' 

 metra studeri, P^ is only slightly enlarged, while in others, as in Cyllometra manca 

 or C. alhopurpurea, IV and also Pg are very considerably modified. 



In the Tropiometridse (figs. 279, p. 213. and 308, p. 223) tlie oral pinnules differ 

 but very little in size or in character from those succeeding, which in this family 

 are much longer than usual. 



In the CalometridsB (figs. 311. p. 223, and 314, 320 p. 227) the proximal pin- 

 nules resemble those of certain sfiecies of Colohometra most nearly, but P^ is much 

 more delicate and is composed of numerous very small rounded segments, and the 

 following pinnules, which may or may not be elongated, are sharply prismatic. 

 The first two segments of the earlier pinnules, especially of the first, are flattened 

 and greatly enlarged with broad dorsal processes, this modification, universal 

 throughout the family, not being found carried to such an extreme elsewhere. 



There are two types of differentiation of the oral pinnules in the Thalasso- 

 metridae. In the genera Ptilometra (figs. 280, p. 213, and 316, p. 227), Pterometra, 

 and Asterometra (fig. 285, p. 215), in which all the pinnules are stiff and spine- 

 like, and the arm bases are so closely crowded against each other that there is no 

 need for any special adaptation of the proximal for protective or for tactile pur- 

 poses, Pj is similar to the succeeding pinnules, but much smaller, and P. is usually 

 also smaller than P3. We see here indications of the course possibly followed in 



