.244 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



discoidal, always cirriferous, varying from thin to thick, the dorsal pole always 

 somewhat smaller than the base, the sloping sides slightly convex; the dorsal pole 

 is usually smooth, sometimes faintly pitted, and is most commonly flat or more 

 or less concave, less frequently, and usually only in small species, more or less 

 convex; the cirrus sockets are large and crowded, and are arranged in from one 

 to three of four (but mostly in one or two) alternating rows; the central cavity 

 of the centrodorsal is comparatively small. In these families the centrodorsal 

 has reached the same stage of development, and is practically the same throughout 

 all the species; it furnishes (except in regard to the excavation for the basal raj's, 

 which will be explained later) no valid systematic characters; though the number 

 of rows of cirri, the comparative concavity or convexity of the dorsal pole, and the 

 occasional markings on its surface are in some cases good specific indices, none of 

 them can be relied upon. Like the size of the central cavity, the size of the cirrus 

 sockets, and the proportionate size of the dorsal pole and consequent angle which 

 the sides make with the base, they are sometimes useful as a supplement to char- 

 acters exhibited by other structures; but at the best they are uncertain, in respect 

 to both generic and specific differentiation. 



In the Thalassometridse (figs. 93, p. 153, 94, p. 155, 95, p. 157, 96, 97, p. 

 159, 187-190, p. 235, 191-198, p. 237, and 199-205, p. 239) and in the Charito- 

 metridse (figs. 99, p. 160, and 206, 207, p. 239) the case is quite different; here 

 the centrodorsal takes on a considerable variety of form and becomes of great 

 importance, both generically and specifically. In the Thalassometridse the centro- 

 dorsal is usually rounded-conical, but less than twice as high as broad at the base, 

 and the lateral surface is usually separated by more or less pronounced interradial 

 ridges into five radial areas, each of which contains usually two, more rarely three, 

 definite columns of cirrus sockets. The dorsal pole is usually small, and, though 

 sometimes flat, is usually ornamented in some way, either pitted or thickly covered 

 with small tubercles or spines, and the interradial ridges and the inferior margin 

 are often similarly ornamented. 



In Ptilometra (figs. 93, p. 153, and 187, 188, p. 235) the centrodorsal is very 

 large, thick discoidal, the sides only slightly oblique, the dorsal pole broad and 

 flat ; the cirrus sockets are arranged in two or three crowded alternating rows in 

 one species, while in the other the rows tend to lie directly under each other, so 

 that the cirri are nearly or quite in 15 columns, three to each radial area, though 

 the radial areas are not in any way marked off, and the columns are closely 

 crowded against each other. 



In Asterometra (figs. 43, p. 77, 94, p. 155, and 189, 190, p. 235) and in Ptero- 

 metra the centrodorsal varies from long conical to columnar, being usually columnar 

 basally with the portion beyond the cirrus sockets conical, the very small polar 

 area with five rounded tubercles which are radial in position; the sides are more 

 or less flattened, and are divided into five radial areas by broad and more or less 

 deep grooves or furrows, each radial area containing two columns of cirrus sockets 

 of from two to (rarely) four each, which are separated from each other by narrower 

 and less prominent (midradial) grooves than those delimiting the radial areas. 

 This type of centrodorsal is essentially like that found in the larger and more spe- 



