98 THE APODOT7S HOLOTHUEIANS 



The Synaptids of this, the largest genus of the family, are of medium or 

 small size and dull color. With few exceptions, they occur only in tropical or 

 subtropical waters and are specially characteristic of the Indo-Pacinc region, 

 where more than three-fourths of them occur. In most cases only one or two 

 individuals of a species are known, so that specific limits are very hard to de- 

 termine, but those herein accepted seem to have characteristic calcareous de- 

 posits. While most of the species occur in shallow water (under 100 m.), 

 seven of them are from depths of over 1,000 m., and all known synaptids 

 from such great depths belong in this genus. Little is known of their habits, 

 and even the appearance in life is unrecorded for the great majority. The fol- 

 lowing key is based of necessity upon the size and form of the calcareous par- 

 ticles. When it is realized, however, that we do not at present know how greatly 

 these may vary, not only in the individuals of a species, but in different parts of 

 one individual, skepticism as to the validity of some of these species is bound 

 to arise, and this is increased by the knowledge that in several cases only frag- 

 ments of the animal, whether anterior or posterior is uncertain, are known. 

 The two New Zealand species described by Hutton ('72), uncinata and in- 

 equalis, are absolutely indeterminable (see Dendy, '97) and are therefore 

 omitted. 



KEY TO THE SPECIES OF PROTANKYRA. 



A. Tentacles 12; entirely marine. 



B. Anchors and plates all of one kind, though they may vary somewhat in size. 

 G. Anchors not conspicuously asymmetrical. 



D. Anchor-plates more or less elongated and irregular in outline, with com- 

 paratively few (20-50) holes; these have smooth margins, and one or 

 more of those near center of plate are conspicuously larger than the 

 others, and are polygonal or elliptical in shape (plate v, fig. 26 ; plate 

 iv, figs. 4 and 5). 



E. Stock of anchor not branched (plate iv, fig. 3) ; accessory calcareous 

 bodies not in the form of elongated slender, straight, or bent rods. 

 F. Plates more or less asymmetrical (plate v, fig. 26). 



Accessory calcareous bodies in the form of small oval, notched 

 or irregular short rods, perhaps sometimes wanting. 



CHALLENGERI 



f 



Accessory calcareous bodies in the form of large, irregular per- 

 forated plates BICORNIS 



FF. Plates nearly symmetrical, with a single pair of large elliptical 

 holes at center and a number of smaller ones at each end 



(plate iv, figs. 4 and 5) DUODACTYLA 



EE. Stock of anchor branched (as in fig. 25, plate iv) ; accessory calca- 

 reous bodies long, slender rods, either straight or bent, often 



notched at the tip SLUITERI 



DD. Anchor-plates more or less oval or elliptical, usually with numerous 

 (40-150) holes, but sometimes with only 20-50; these are more or less 

 circular, are often dentate, and near center of plate are usually some- 

 what larger than near margin (plate iv, figs. 9 and 13). 



