290 BULLETIN 82, UNITED STATES NATIONAL MUSEUM VOLUME 1 



or more or less expanded; in F. mawsoni some fenestrated plates may be present near 

 the tips of the pinnules. In Heliometra the deposits may be similar, with the ends of 

 the rods, especially the inner, expanded and pierced with one or two holes, the rods in 

 some specimens broadening out into irregular plates, or even transforming into well 

 marked covering plates or partially differentiated covering and side plates. In Antho- 

 metra the lateral perisome of the pinnules is completely enclosed in a continuous distally 

 imbricating series of highly developed plates. In Promachocrinus similar plates may 

 occur, or the perisome may be quite without calcareous deposits (see vol. 1, part 2, 

 pp. 268-271; also John, Discovery Reports, 1938, pp. 143-144). 



In Promachocrinus the tentacles contain a conspicuous continuous band of fine 

 interlaced spicules which runs along the outer side nearly to the tip, and irregular 

 spicules are occasionally to be found in the tentacles of Heliometra and Florometra. 



Relationships oj the genera. It is possible to arrange the 5 genera included in the 

 Heliometrinae in a linear series on the basis of the relative degree of specialization of 

 the oral pinnules, the centrodorsal and the brachials. 



The minimum amount of specialization hi the oral pinnules is found in Anthometra, 

 in which only P t is a typical oral pinnule, P 2 being about half as long with less than half 

 as many elongated segments. The centrodorsal of Anthometra also retains the original 

 conical form only slightly, if at all, modified. The carination of the elements of 

 the IBr series and first two brachials more or less characteristic of the young of all the 

 other genera is retained throughout life in Anthometra and becomes greatly accentuated, 

 being carried out onto all the brachials, which have slightly serrate or spinous distal 

 ends. Anthometra is further peculiar in being of rather slender build, with the arm bases 

 making a relatively small angle with the dorsoventral axis, and in having relatively 

 few very long, rather slender, and only slightly curved cirri, the longest of which, how- 

 ever, have a very large number of segments. Although Anthometra is emphatically a 

 member of the Heliometriuae, it is the least characteristic of all the genera, and shows 

 the nearest approach to the Bathymetrinae. 



In Promachocrinus the relative shortness of the arms, which have half the number 

 of brachials common to the other types, and the irregular distribution and relative 

 infrequency of the syzygies, are features correlated with the occurrence of 10 instead 

 of 5 arm pairs, if we may judge from comparison with other multibrachiate types. 

 The possession of 5 additional radials interpolated between the 5 normal "radial radials" 

 gives the species of this genus an entirely different appearance from that of all the other 

 genera; but this apparent isolation is not supported by any other features, and the 

 10- or 12-armed individuals of Promachocrinus kerguelensis which are sometimes found 

 are most difficult of determination. 



Sometimes in Promachocrinus PI is the only oral pinnule, as in Anthometra, though 

 P 2 may be similar to it and is usually but little shorter. The centrodorsal is always 

 sharply conical. The arm bases make a relatively small angle with the dorsoventral 

 axis as in Anthometra, and the brachials always have spinous distal ends. The cirri of 

 Promachocrinus are rather slender, and are little curved. 



In spite of the utterly different appearance it is impossible not to recognize the 

 fundamental similarities between this genus and Anthometra, over which it represents 

 a relatively slight advance in the direction of pinnule specialization. 



In Solanometra, Heliometra and Florometra, fully developed individuals have at 

 least PI, P 2 and P 3 modified as typical oral pinnules. Solanometra, with a hemispherical 



