246 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



cializcd species of Thahft.tometra (such as T. gigantea) (fig. 203, p. 239), and is the 

 most perfected derivative from the thalassometrid line of development. 



In Thalassometra, Agalaometra, and Horseometra (figs. 195-198, p. 237, and 203, 

 p. 239) the centrodorsal is comparatively small and conical, wnth the lateral sur- 

 face di^aded into five radial areas by low rounded ridges, wliich are sometimes sup- 

 plemented by five similar but smaller ridges in the midradial line; these ridges are 

 the result of the resorption of the surface of the centrodorsal, which ]irogresses 

 much faster in the radial areas than in tlie interradial areas, and thus leaves the 

 latter standing out as more or less prominent ridges; the cirrus sockets are arranged 

 in 10 (very rarely l.*)) columns of two or three each, these columns being always close 

 to the interradial ridges and often more or less separated interiorly, possibly as a 

 result of the suppression of a primitive median column; occasionally a more or less 

 complete third column is found in this midradial gap. The small dorsal pole is 

 usually tubercular or fmely spinous, and the interratlial ridges and mferior margin 

 are also commonly spinous. The central cavity appears large, but when the pro- 

 l)ortionately small size of the centrodorsal as a whole is taken into consideration 

 it is found to be in reality relatively small. 



Stylometra (figs. 193, 194, p. 237) and Crotalometra have centrodorsals resem- 

 bluig those of Thalassometra; but that of Stylometra is rather more spinous, 

 especially at the dorsal pole, than those of any species of Thalassometra, wliile that 

 of most of the species of Crotalometra is rather larger, smooth, and more definitely 

 conical, sometimes being more or less columnar basally, like that of Asterometra. 



In Stenometra (figs. 191, 192, p. 237) the centrodorsal is small, truncated conical 

 or more or less columnar, with the interradial ridges usually ver\' strongly developed 

 and supplemented by radial ridges, which are sometimes verj'' prominent; the cirrus 

 sockets are arranged in 10 definite and well separate<l columns of two or three each. 



Stiremetra (fig. 205, p. 239) has the centrodorsal small, hemispherical or bluntly 

 conical, the dorsal pole more or less papillose; the cirrus sockets are arranged in 

 two or three columns of one or two each in each radial area, though the columns 

 are not especially ililTerentiated. 



The centrodorsal of Cosmiometra (fig. 204, p. 239) is essentially lilvc that of 

 TJialassometra, but it is usually more rounded, the sides maldng a rather greater 

 angle with each other, and the dorsal pole being proportionately smaller; the ratlial 

 ridges also not so well marked. 



Parametra (figs. 199-202, p. 239) has a proportionately larger and broader, though 

 lower, centrodorsal than anj'' other genus in the family; it is low hemispherical or 

 more or less discoidal, with a broad dorsal pole, instead of inclining to conical as 

 usual. The cirrus sockets show more or less irregularity of arrangement, but are 

 usually in two rows, and approximately in 10 or 15 columns. Taken as a whole, 

 the centrodorsal of Parametra is much more like the type prevailing in (he Charito- 

 metridaj than like that found in the Thalassometridse, and the short, comparatively 

 stout, cirri help to increase the resemblance. 



In the Charitometrida} (figs. 99, p. 160, 100, p. 162, 101, 102, p. 163, and 206, 207, 

 p. 239) the centrodorsal is broad, and varies from thin discoidal to thick tliscoidal 



