94 THK APODOUS HOLOTHUKIANS 



LABIDOPLAX Ostergren, i8g8b. 



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

 ing. Polian vessel usually single. Stone-canal single. Stock of anchor 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 nc means certain that the second of 

 these would not be associated more naturally with Protankyra. 



KEY TO THE SPECIES OP LABIDOPLAX. 



A. Anchor-plates with 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 12, with 4 digits MEDIA 



AA. Anchor-plates with numerous, irregular, smooth holes (plate v, 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 1 50-300 p.; anchor-plates with few small marginal holes 



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



BB. Anchor-plates, at least anteriorly, very wide, i. e., 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, PIG. 23. 



Synapta buskii Mclntosh, 1866, p. 612. Anchor and plate, fig. 6. 

 Synapta tencra Norman, 1864 (no description). 

 Labidoplax tenera Ostergren, 1898&. 

 Labidoplax buskii Ostergren, 1903. 

 Labidoplax buskii Ostergren, 1905a, p. CLVI. 



LENGTH. 15-30 mm., with diameter about one-tenth as much. 

 COLOR. Pale, transparent grayish, without pigment. 



DISTRIBUTION. Reported from Outer Hebrides (Mclntosh) ; British Isles 

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



