274 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Beyond each of these side plates in the ambulacral lappets, but in contact Avith 

 their outer ends, lies a considerably smaller covering plate which is irregular in 

 shape, but usually more or less elongate, and tapers to the point of contact with 

 the side plate. 



In addition to this type Mortensen figured very long and extremely narrow 

 band-like plates standing at right angles to the pinnulars, with beyond them, im 

 the ambulacral lappets, verj' irregular but usually narrow and elongate covering 

 plates. 



Isometra vivijyara (figs. 1328, 1331, pi. 49).- — Mortensen states that " some thin, 

 irregular fenestrated plates are developed along the borders of the ambulacral fur- 

 rows. They represent the side and covering plates, the latter being the larger. 

 The side plates are often irregularly arranged, so that it is difficult to find the one 

 corresponding to each covering plate; they may be even totally wanting; also, the 

 covering plates may be reduced to a single spicule, at intervals even totally 

 disappearing." 



Mortensen has figured the spicules in the tentacles of this species; they are 

 long and straight, with usually rather numerous short and usually blunt pro- 

 jections. They are very inconstant. Mortensen says that " sometimes there may 

 be quite a bundle of them in a single tentacle of a pinnule, the rest of them being 

 entirely devoid of spicules." 



JRathrometra prolixa (figs. 1170, pi. 27, and 775, p. 362). — In the specimen ex- 

 amined each of the ambulacral lappets contains a very long, more or less evenly 

 curved, smooth rod, the outer end of which usually has a few minute projections, 

 or. more rarely, is slightly forked or expanded, with a few perforations. 



There were no spicules in the tentacles. 



Mortensen has figured two types of plates in the lateral perisome of the pin- 

 nules of this species. 



In one type the plates are in the form of long, smooth, curved rods, of which 

 the outer end, lying in the ambulacral lappets, is expanded and narrowly fan- 

 shaped, with from four to nine perforations and an unfinished and irregular distal 

 border, and the inner is parallel to the border of the pinnular, which it follows 

 proximalty to well beneath the recumbent portion of the succeeding rod. 



In the other type the rods are thicker, Avithout any broadening of the outer end, 

 abruptly bent in an obtuse angle in the middle, and studded with minute thorns. 



In a pentacrinoid larva figured by Mortensen these rods are more irregular, 

 with one or two long processes from about the region of the median angle on the 

 convex (distal) side. 



Mortensen figures occasional spicules in the tentacles of the young penta- 

 crinoids of this species. These are in the form of irregular elongated calcareous 

 rods, which bear frequent minute obtuse spines. 



riathrometra teneUa. — Each ambulacral lappet contains a very slender and 

 delicate smooth rod, usually slightly curved, and of moderate length or short. 



Some of the tentacles contain a thin line of spicules, which runs for a short dis- 

 tance along the outer side. 



