A MONOGRAPH OF THE EXISTING CRINOIDS 183 



Indeed, it is probably more closely allied with them than they are with other species 

 included within the same genus. But there is a more definite break between Craspe- 

 dometra and Heterometra tban occurs anywhere within the species groups included in 

 the latter genus. 



The single species of Homalometra, like that of Craspedometra, is merely an 

 offshoot from one of the several genetic lines included in the genus Heterometra; but 

 there is a considerable gap between Homalometra and the most closely related species 

 of Heterometra. 



The eight species and forms included in the genus Amphimetra form a closely 

 knit unit that is rather abruptly distinct from the other types included in the family 

 Himerometridae. Though in its arm structure, particularly in the very short bra- 

 chials, Amphimetra closely approaches one of the groups within the genus Heterometra, 

 and also the genus Himerometra, it shows certain features which indicate an approach 

 to the Colobometridae. Thus through individual variation specimens of Amphimetra 

 tessellata tessellata develop IlBr series, which are always of 2 elements and not 4(3+4) 

 as in the other genera of Himerometridae, and other specimens have the dorsal 

 processes on the cirrus segments much broadened and somestimes forked. 



Like the species of the Zygometridae, the species of Himerometridae are espe- 

 cially characteristic of the region from southern Japan to northern Australia and the 

 eastern portion of the Bay of Bengal. One genus (Amphimetra) extends westward 

 to the Maldive archipelago, one (Himerometra) is represented by a single species in 

 the Persian Gulf, and a third (Heterometra) is represented by several species on the 

 east African coast. 



All the genera but one (Homalometra) are represented at the low-tide mark, and 

 the included species are chiefly to be found in very shallow water. None of the 

 species have been dredged from a greater depth than 111 (or possibly 183) meters. 



History.— The family Himerometridae was first mentioned in my paper on new 

 genera of unstalked crinoids published on April 11, 1908, as a simple heading under 

 which the new genus Oligometra was described. Later in the same paper the genera 

 of free crinoids belonging to the Comatulida were grouped in several families, which 

 were not defined. Among these was the family Himerometridae, including the genera 

 Himerometra, Pontiometra, Cyllometra, and Oligometra. 



In a paper of mine published on May 14, 1908, a key to the families of Comatulida 

 having recent representatives was given, in which the family Himerometridae was 

 included. The family Himerometridae was said to contain the genera Oligometra, 

 Cyllometra, Himerometra, and Pontiometra, and its range was given. 



In my paper published on October 30, 1908, the arm structure and brachial 

 homologies of the family Himerometridae were discussed in detail. 



In a revision of the family Himerometridae published on January 9, 1909, the 

 following new genera, including species previously assigned to the large and hetero- 

 geneous genus Himerometra, were added to it ■ — Colobometra, Amphimetra, Cenometra, 

 Craspedometra, Stephanometra, Heterometra, and Dichrometra — and the new genus 

 Oxymetra was mentioned but not defined. 



In a paper on the new genus Comatilia published on April 27, 1909, 1 remarked that 

 I had become convinced that the division of the recent comatulids into two contrasted 

 groups, called by Carpenter Antedon and Actinometra, was very artificial and could 



