224 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



pinnules tliere is a double row of large plates which are rectangular at the base 

 but somewhat more irregular in shape at their upper ends. There are generally 

 five or six plates in each row, but those of the two sides have no fixed relative 

 positions, sometimes corresponding exactly and sometimes alternating as exactly. 

 These are best developed in Pcecilomctra (figs. 1057, 1058, 1061, pi. 14). In the 

 other genera of the Charitometridse they are smaller and much more irregular, 

 while those of the two sides may be separated by the ambulacrum with its well- 

 developed side and covering plates. 



In Pcecilometra, and in such forms as Periasonutra angusticalyx (fig. 1055, 

 pi. 14) , these swollen genital pinnules receive no branches from the brachial ambu- 

 lacra, resembling in this respect the corresponding pinnules in many of the Comas- 

 teridae, and the anambulacral plates covering the gonads consequently meet one 

 another in the midventral line of the pinnule. The sacculi, however, which lie 

 at the sides of the ambulacra, may extend onto these grooveless pinnules and 

 occupy small holes between the large protecting plates, while on the outer seg- 

 ments of the pinnules, beyond the gonads, the sacculi occupy the median groove 

 on the upper surface of the skeleton. 



The presence of this plating over the gonads is not, as was supposed by 

 Carpenter, confined to those species in which the side and covering plates are 

 highly developed, but it may occur in any group. It is frequently highly developed 

 in Heliometra and in Pentametrocrinus. 



Plating ol tin- disk. 



Under ordinary examination the disk may appear quite naked, or it may be 

 more or less covered with calcareous nodules or plates of various sizes which 

 sometimes form a solid pavement over it. The presence or absence of a calcareous 

 plating is of considerable importance from a systematic standpoint. 



Strictly speaking, the disk always contains limy deposits, even though they 

 may be invisible exteriorly and microscopic in size. In certain species they are 

 present in the form of very minute spicides and imperforate plates and are con- 

 fined to the sides of the ambulacral grooves, but usually they are more or less 

 uniformly distributed throughout the integument of the disk showing, however, 

 a strong tendency to become segregated, or at least most strongly developed, along 

 the ambulacra and in the middle of the interambulacral areas, especially dorsal 

 to the periphery of the ventral face. Almost invariably the calcareous deposits 

 are more numerous and larger in the anal area than elsewhere, especially about 

 the base of or on the anal tube, and frequently this is the only part of the disk 

 where they are visible. 



Speaking broadly, it may be said that among the macrophreate forms the disk 

 is without noticeable calcareous deposits, while they are usually more or less 

 evident in oligophreate species. There ai'e, however, very numerous excejitions 

 to this generalization. 



Except for the side and covering plates the perisomic plates are usually 

 confined to the disk, or at most do not extend beyond the outermost division 



