292 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



metridse, Mariametridse (fig. 344, p. 287), and Stephanometridas (fig. 340, p. 287), the 

 structure of the cirri appears to be quite comparable to that of the Charitometridas, 

 and transition segments appear never to have occurred. On the other hand, in 

 a single genus of the Macrophreata, Zenometra, a more or less marked transition 

 segment is found, comparable in every way to that of certain of the Oligophreata. 



There is a curious correlation between the cirri and the proximal pinnules; 

 species in which the latter are large, as Craspedometra acutitirra, as a rule have long 

 cirri with numerous segments, while species in which they are not especially de- 

 veloped, as Heterometra quinduplicava, commonly have cirri with fewer segments. 

 Again in certain species, as in Oligometra sempinna and in the species of Prometra, 

 there may be more or less variation in the number of segments in the enlarged 

 proximal pinnules; this is found upon examination commonly to agree directly 

 with a similar variation in the number of cirrus segments. This correlation is most 

 marked and most obvious in the Comasteridas. In this family species with large and 

 very long proximal pinnules which are stout basally, like Comanfhus "bennetti or 0. 

 pinguis, have very large and stout cirri with a large number of segments, while 

 species with a few small and weak cirri, or none at all, as ComantJius annulata or C. 

 parvicirra, or many of the species of the genus Comaster, have the proximal pinnules 

 small. 



This interrelationship between the cirri and the proximal pinnules appears to 

 be confined to the Oligophreata, and in this group it is of more or less uncertain 

 occurrence, being by no means general. 



There is a closer and more widespread agreement between these two sets of 

 structures in regard to the modification of the distal ends of the component segments, 

 an agreement which is further correlated with a similar modification of the ossicles 

 of the calyx, the division series and the arm bases. In cases where, as in Thalas- 

 sometra villosa, Stylometra spinifera, or in the species of the genus Colobometra, the 

 distal ends of the cirrus segments are produced and spinous, the calyx and arm bases, 

 as well as the distal edges of the segments of the proximal pinnules, will also be found 

 to be spinous, though this spinosity is less, and may be entirely suppressed on the 

 brachials, from the fourth onward, and on the genital and distal pinnules. This 

 type of correlation is not found outside of the Oligophreata, except in the genus 

 Zenometra. 



The striking correlation, both in structure and in function, between the cirri 

 and the ungrooved pinnules in Comatuletta 'brachiolata has already been discussed 

 in detail. 



Mention must also be made of the curious case illustrated by the families 

 Thalassometridae, Charitometridas and Tropiometridse. In the Charitometridae 

 and Tropiometridas smooth and very stout cirri accompany very slender many 

 jointed proximal pinnules; the long and spiny cirri of the Thalassometridas occur 

 together with greatly enlarged, swollen, and elongated proximal pinnules, the 

 accentuation of these characters in the latter being to a considerable degree cor- 

 related with the proportionate length of the cirri. 



Though in Asterometra, Pterometra and Ptilometra (which together form the 

 subfamily Ptilornetrinse) the cirri are excessively long, and are in structure just like 



