94 THE APODOTJS HOLOTHURIANS 



LABIDOPLAX Ostergren, 1898b. 



Tentacles digitate, 11 or 12, with 3 or 4 digits. Cartilaginous ring want- 

 ing. Polian vessel usually single. Stone-canal single. Stock of anclior toothed 

 but not branched; arms usually toothed; vertex smooth. Anchor-plates with 

 posterior end abruptly narrowed (forming a sort of handle), the bow crossing 

 its outer surface rudimentary or wanting. 



This small group of synaptids stands midway between the preceding and 

 the following genus and doubtless indicates the line of connection between the 

 two. They are confined to the Old World, but range from the Arctic Ocean to 

 the tropics. As will be seen from the followirg artificial key, the genus falls 

 naturally into two sections, and it is by no means certain that the second of 

 these would not be associated more naturady with Protankyra. 



Key to the Species of Labidoplax. 



A. — Anchor-plates witli 7 large, more or less dentate holes in the free end, arranged as in 

 Euapta or Synapta (plate v, fig. 23). 



Tentacles 11, with 3 digits buskii 



Tentacles 13, with 4 digits media 



AA. — Anchor-plates with numerous, irregular, smooth holes (plate y, fig. 25) ; tentacles 12, 



with 4 digits. 



B. — Anchor-plates, even anteriorly, usually much longer than wide, i. e.. width less than 



.60 of length ; width of anchor-arms from tip to tip less than .70 of length of 



anchor and usually under ..55; sense-cups commonly present on inner face of 



tentacles. 



Anchors posteriorly 300-500/* long; anchor-plates with numerous small holes 

 near margin, especially at posterior end, not showing any special arrange- 

 ment DIGITATA 



Anchors posteriorly 150-300 fi ; anchor-plates with few small marginal holes 



and about 8-10 large holes arranged in 3 or 4 transverse rows dobia 



BB. — Anchor-plates, at least anteriorly, very wide, i. c, width usually more than .70 of 

 length; width of anchor-arms usually. 70, sometimes over .80, of length of anchor; 

 no sense-cups on tentacles thomsonii 



Labidoplax btjskii. 



Plate V, Fig. 23. 



Synapta buskii Mcintosh, 1866, y>. 612. Anchor and plate, fig. 6. 

 Synapta tencra Xorman, 1864 (no description). 

 Labidoplax tenera Ostergren, 18986. 

 Labidoplax buskii Ostergren, 1903. 

 Labidoplax buskii Ostergren, 1905(7. p. clvi. 



Length. — 15-30 nun., with diameter about one-tenth as much. 

 Color. — Pale, transparent grayish, without pigment. 



Distribution. — Eeported from Outer Hebrides (Mcintosh) ; British Isles 

 (Norman) ; west coast of Sweden (Theel) ; and coast of Scandinavia, from Cat 



