PART 3 A MONOGRAPH OF THE EXISTING CRINOIDS 651 



Belationships oj the genera. — In the subfumily Bathj-metrinae the characters by 

 which the genera arc differentiated are distributed among the oral pinnules, the cirri, 

 the centrodorsal, the genital pinnules and the brachials; in other words, the members 

 of this subfamily are unusually well balanced forms and do not show one or two sets 

 of structures highly diversified while others remain generalized, as do all the other 

 comatulid groups. 



There is a great temptation to consider the Bathymetrinae as a subfamily including 

 generahzed or "primitive" types and forming a complex from which the other sub- 

 famihes of the Antedonidae have been derived. Thus the Heliometrinae might seem 

 to be an offshoot from the Bathymetrinae near the genus Boleometra; the Zenometrinae, 

 as is shown by the young of Psathyrometm mira (see p. 525) pass through a bathymetrine 

 stage in their development suggesting certain species of Thaumatometra besides which 

 Poliometra and Hathrometra have much in common; the Thysanometrinae recall 

 Thaumatometra tenuis; the Perometrinae are more or less similar to a small group of 

 genera within the Bathymetrinae; and there is no very sharp line of demarcation 

 between the Antedoninae and the Bathymetrinae. 



Yet, heterogenous as the subfamily Bathj^metrinae appears to be, there is a certain 

 uniformity of structm-e throughout the group whereby any specimen may at once be 

 recognized as belonging to it, and the diversity shown in all the structural features 

 may better be interpreted as indicating the most virile and plastic of all the comatulid 

 groups and the one which, as a result of its plasticity, has been able to adapt itself to the 

 greatest ecological range, rather than as indicating anj- phylogenetic inferiority. 



Standing somewhat apart from the rest of the Bathj^metrinae are 2 genera, 

 Thaumatometra and Bathymetra, in which the cirrus segments are much elongated and 

 the cirri much compressed laterally. These hve on ooze or mud in deep still water, 

 and their higldy specialized cirri are well adapted to such a habitat. Bathymetra 

 has been assumed to be a very primitive type on account of its long radials; but the 

 length of the radials results from the specialization of the centrodorsal which increases 

 in length instead of in breadth, and this, taken in connection with the curious cirri, 

 shows that it cannot be considered more primitive than any other genus in the group. 



The species of Thaumatometra vary from species approaching those of Bathymetra, 

 such as Th. septenirionaiis and Th. ahyssorum, to species like Th. isis, which approach 

 the species of Trichometra and allied genera. 



The occurrence of a marsupium on the female genital pinnules in which the young 

 develop is an interesting feature of the genus Phrixometra, which otherwise is in no way 

 remarkable. 



Great elongation of Pi, which is slender and hairlike, but stiffened, and composed 

 of numerous segments, while Po resembles Pj, characterizes Hathrometra and Retiometra. 

 In the former the centrodorsal is conical, as in Fariometra, while in the latter it is hemi- 

 spherical, as in Trichometra and other related genera. In Retiometra P2 is a genital 

 pinnule, as in certain species of Thaumatometra and other allied genera, while in 

 Hathrometra it is not. 



In Boleometra and in Thaumatometra tenuis, Pj has numerous segments of which 

 the basal 5 to 7 are not longer than broad and have the corners cut away as in the 

 corresponding pinnule in the Heliometrinae. In Thaumatometra tenuis Pj, which is 

 only a httle shorter than P,, is a genital pinnide, but in Boleometra it is distinctly 

 shorter than P,, though not a genital pinnule. 



