198 LAXD AND FRESHWATER 



The type of the subgenus Si/l-esia is dathratula, Pfr.,from Ceylon, 

 and as the shell of this species is so very distinct in form from that 

 of T. guerini, the type of Tliysanota, I follow Gude and retain it. 

 I certainly consider, however, that systematically (speaking from 

 what its anatomy has shown me) it has the closest relationship 

 with Thi/sanoUi and must be placed as a subsection of that genus 

 in preference to that of PJectopylis, yet forming at the same time 

 a natural link with this last genus. 



The great distinctness as a group of the Southern Indian and 

 Ceylon species of Plectojxjlis is pointed out by Mr. Gude. Dealing 

 with the geographical distribution of the genus, he says (' Science 

 Gossip,' vol. vi, Nov. 1899) : — " A wide gap separates the Sikkim 

 forms from the South Indian and Cingalese species, a fact which 

 will be less surprising, if, as I suspect, the latter prove to belong to 

 a distinct genus." Again, in the same periodical, vol. vi. p. 149, 

 when writing on new sections of Plectopt/lis, he states -. — " I strongly 

 suspect that when the anatomy of the Philippine species (section 

 Enteroplax) is investigated, the group will be found to differ so 

 widely from typical Plectopiilis that it will have to be raised to the 

 rank of a separate genus. The same may prove to be the case with 

 the section Sykesia." 



Original description : — " Testa perforata, convexo-lenticularis, 

 tenuis, pellucida, cornea, oblique plicata, hicarinata ; cannis pilis 

 lonyis ciliatis ; spira parum elevata, vertici suhtili; anfr. 4^ scalares, 

 ultimiis antice non descendens, hasivix convexus; apertura subobliqua, 

 depresse securiformis ; perist. simplex, rectum, maryinibus subparal- 

 lelis, columellari vix rejJexiusculo. 



" Diam. major 7i, min. 6|, alt. 3| mill. {Coll. Skinner). 



" Habitat in insula Ceylon." 



Animal preserved in formalin. The whole foot was in extended 

 state outside the shell. As these very small animals are white and 

 pellucid, and, to a certain extent, contracted, it is by no means easy 

 to make out what their form presents in a living state. The foot is 

 long and narrow with a broad margiu folded down the centre. No 

 mucous pore could be detected. The shell-lobe borders and over- 

 laps the peristome on the left side. There is a very pronounced 

 sliell-muscle. The tentacles are invaginated. 



The jaw was distinctly seen in profile. I did not venture to 

 meddle with it, for fear of losing it altogether and what was near it. 



The radula (fig. 3 b) is similar to that of Philalanka : — 



13 . 2 . 9 . 1 . 9 . 2 . 13 

 25 . 1 . 25 



The middle tooth is on a narrow oblong plate, its point reaches 

 halfway up, with very minute basal cusps. 



The 1st admedian teeth are also on plates higher than their 

 breadth and have one long tooth and one remote small side cusp. 

 The 10th and 11th are transition teeth, having two short cusps on 

 the outer side. The laterals commencing at the 12th tooth are on 



