PART 5 A MONOGRAPH OF THE EXISTING CRINOIDS 651 



Relationships of the genera. In the subfamily Bathynietrinae the characters by 

 which the genera are 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 

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

 families 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 Psathyrometra 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 tennis; 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 Bathynietrinae appears to be, there is a certain 

 uniformity of structure 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 any phylogenetic inferiority. 



Standing somewhat apart from the rest of the Bathymetrinae are 2 genera, 

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

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

 and their highly 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. septentrionalis and Th. abyssorum, 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 P], which is slender and hairlike, but stiffened, and composed 

 of numerous segments, while P 2 resembles P 3 , 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 P 2 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, P! 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 P 2 , which is 

 only a little shorter than P,, is a genital pinnule, but in Boleometra it is distinctly 

 shorter than P,, though not a genital pinnule. 



