322 A. JE. Verrill — North American Ophiuroidea. 



or the number of joints that have two pairs varies in some species 

 according to age, the number of these parts and of the spines in- 

 creasing in the older specimens. In very large specimens of small 

 species there is often a tendency to develop extra oral-papillae and 

 tooth-papilIa3, either above or below the regular series. The precise 

 number of species cannot be considered as constant in any species, 

 and must always be understood to vary within more or less definite 

 limits. This character has been used in some of the analytical tables 

 only because of its easy observation. The degree to which the 

 larger basal rows of spines appi'oximate dorsally is of more impor- 

 tance, though not invariable, and the character of the serrulations 

 or thorns on the spines is of considerable value, though slightly 

 variable, even in adult specimens. The spines are always rougher in 

 the young specimens. 



Moreover, in using the analytical tables, it must be remembered 

 that some of the species have been described only from a single 

 specimen* and that the amount of variation is still unknown, in cer- 

 tain deep-sea species, which have not yet been studied from the later 

 and larger collections. 



It is also to be noted that in the case of deep-sea species, especially' 

 those obtained by the "tangles," many of the delicate parts are 

 liable to be broken or torn off, and in the case of tentacle- scales and 

 oral papilliB they may leave no traces. When such parts are 

 rej)roduced they may not appear in the same number or form as at 

 first. 



Such accidents may account for many cases where the different 

 arms or different jaws of a single specimen present variations in 

 their appendages,f as well as for specimens in which all the arms 



* Mr. Lyman's custom was to describe all his new species from a single type 

 specimen. Had his health remained unimpaired he would, doubtless, have re- 

 vised more fully the large collections from the later Blake Expeditions. 



fin a large lot of typical Ophiacantha hidentata one abnormal specimen shows 

 curious variations in the mouth-parts, which may be due to the repair of dam- 

 ages. The number of regular oral papillae on the different jaws varies from 

 three to five. On some jaws there is a rudimentary, wart-like, distal one ; in 

 others it is as large as the next ; on one jaw there is an extra, slender, clavate 

 papilla, back of the first, on the lower face of the jaw ; the distal papilla is 

 thick, blunt, clavate, and usually somewhat triquetral ; one jaw has two papilla? 

 grown together for half their length ; one has an extra papilla above the inner 

 one, and of the same form. The tooth-papilliie vary in form and size, and from 

 one to three in number ; one jaw has a terminal pair ; and on one jaw some 

 of the teeth are split into two. The first arm-plate has a vertical process on 



