148 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Atelecrinus, in which genus all of the earlier pinnules are absent. In the remaining 

 genera (figs. 283, 284, p. 215, and 317, 318, 321-323, p. 227) P^ is always more or 

 less, and typically enormously, enlarged, and much longer than the succeeding 

 pinnules, but in the distal half it tapers, usually rather suddenly, to a slender 

 and delicate tip. Pa is very rarely and P„ never similarly modified. Tliis com- 

 bination of defensive and tactile functions is similar to that seen in P, and some 

 of the following pinnules in such genera as Craspedometra, M ariametra, Dichro- 

 Tnetra, Lam-provietra^ Heterometra, Amphimetra, and Decametra, but in the 

 ThalassometrinsB alone does it occur on Pj to which pinnule it is in this group 

 confined. 



In the Charitometridse (figs. 302, 306, p. 223) P^ and P, are very slender with 

 numerous short segments, and are usually slightly shorter than the succeeding 

 pinnules, while P^ is slightly shorter than P,. They form typical tactile pinnules 

 of the type common to most of the oligophreate genera in which tactile pinnules 

 occur, the only difference being that the earlier segments are slightly enlarged, and 

 their outer edges are sometimes more or less flattened as a result of the close 

 compression of the arm bases. Thus in the Charitometridse two pairs of slender 

 oligophreate tactile pinnules are found instead of only one, as in other forms, a 

 condition finding a parallel in the macrophreate geiuis Leptonietrn (fig. 290. p. 221). 



The pinnules in the Macrophreata, oral as well as middle and distal, are 

 usually cylindrical, in section aproximately circular except for the notch indicat- 

 ing the groove accompanying the ambulacral structures; but they may be some- 

 what flattened, as in Psathyrometra (fig. 295. p. 221) and Zenometrn, the section 

 then becoming more or less oval or elliptical. Such a condition, however, is mostly 

 confined to the lower pinnules. 



In the Oligophreata the oral pinnules are invariably primatic, at least in 

 part, and this condition may extend to all the pinnules, as in the Calometridae 

 (figs. 334, p. 227, and 336. 339, p. 229), Thalassometridse (figs. 332, p. 227, and 337, 

 338, 343, 344, 350, p. 229), and Charitometridse, or to the middle pinnules but 

 not the distal, as in the Tropiometridse. In the Comasteridse this feature is 

 generally observable in the segments forming the terminal comb, and sometimes 

 it extends inward nearly or quite to the pinnule base. In the species of Zygo- 

 metridse and Himerometridse there is usually to be seen on the enlarged lower 

 pinnules a prominent ridge just distal to the dorsal median line which is especially 

 prominent on the basal segments. This ridge is especially evident in Craspe- 

 dometra, in certain species of Heterov^etra (figs. 268, 269, p. 207), and particularly 

 in Eudioci-inus (fig. 267. p. 207). In the Mariametridae and SteplianonietridsE. 

 as in the Colobometrida?. it is more or less obsolete, though a trace of it may 

 usually be detected on the third segment. In Heterometra crenulata, H. produeta, 

 OUgometra sen^pinna (fig. 315, p. 227), and O. carpc7iteri, in which strong lateral 

 processes are found on the pinnulai-s, these are composed either of the produced 

 ventral edges of the pinnulars or of a great development of the dorsal ridge 

 (sometimes of only the distal part) and usually of both. In the species of Colobo- 

 metrida? having fringes of spines on the distal borders of the pinnule segments 



