MOLLUSCA OI' THE ' CHALLENQ-ER ' EXPEDITION. 319 



DESCRIPTION OF PLATE XIII. 



Fig. 1. The septa, base, and synapticiila of Pachyseris speciosa. Magnified. 



2. Diagram of section across colline : a, synapticular wall ; b, synapticula ; 



c, spaces between tabulate processes of columella. 



3. Mcsandroseris Bottcs. Septa and synapticula. Magnified. 



4. The free top of a septum, showing warty ornaments. Magnified. 



5. Trabeculse, side view, granules and synapticula. Magnified. 



6. Different sections of synapticula. Magnified. 



7. Outline of septo-costa of Mcsandroseris, natural size. 



8. Oblique view, showing synapticula fractured on the side of the septum. 



9. Diagram of a young septo-costa with mural tissue on it and synapticula 



on either side. 



10. Synapticula between processes of the columella. 



11. Synapticula fused to form a short wall. 



12. Synapticula, from above. Magnified. 



MOLLUSCA OF H.M.S. ' CHALLEIir&ER ' EXPEDITION". — Part XIX. 



By the Eev. Eobekt Boo& Watson, B.A., F.E.S.E., F.L.S. 



[Published by permission of the Lords Commissioners of the Treasury.] 

 [Read May 3, 1883.] 



Earn. BiTLLiD^, Stol. 

 Cylichna. 



1. Cylichna discus, n. sp. 



2. tahitensis, n. sp. 



3. crispula, n. sp. 



4. noronyensis, n. sp. 



5. reticulata, n. sp. 



6. Cylichna subreticulata, n. sp. 



7. labiata, n. sp. 



8. (Volvula) paupercula, 



n. sp. 



9. ( ) sulcata, n. sp. 



1. CTLiCHisrA DISCUS, n. sp. 



St. 24. Mar. 25, 1873. Lat. 18° 38' 30" N., long. 65° 5' 30" W. 

 Culebra, St. Thomas, Danish "W. Indies. 390 fms. Coral-mud. 



Shell. — Narrowly cylindrical, truncated in front like C. alba ; 

 brown, with a small disk-like minutely perforated top, smooth, 

 thin, polished. Sculpture. Longitudinals — there are very faint 

 and slight lines of growth. Spirals — there are some very doubtful 

 indications of a coarse spiral structure in the texture of the shell. 

 Golour milky white and glossy. Mouth the full length of the 

 shell but not more, extremely narrow, with lips nearly parallel 

 till they diverge in front, the inner lip following the slow basal 



23* 



