SOWEEBY : ON PLESIOTROOHVS AND N.SPP. OK MOLLUSCA. 215 



Plesiotrochus ceylonicus, n.sp. 



Testa parva, iraperforata, conica, albida ; spira data, acuta, 

 anfractus 6, apicales 2 Iseves, ca^teri planato-declives, infra acute 

 unicarinati, spiraliter exiliter striati, hie illic longitudiualiter 

 irregiilariter pauciplicati, sutura impressa minute cienulata sejuncti; 



anfractus ultiiims f longitiidinis testae sequans, leviter ventricosus, ad 

 peripheriam bicarinatus, infra concavus ; apertura lata, antice 

 breviter canaliotilata; columella leviter contorta ; labrum tenue, 

 arcuatum. Long. 3, diani. maj. 2 mra. 



Hah. — Ceylon. 



Compared with P. pagodiformis, Hedley, this shell is smaller, less 

 elately conical, less longitudinally plicate, whorls less concave, and 

 base more ventricose. 



Referring to Hedley's interesting comments on this genus (Proc. 

 Linn. Soc. N.S.W., vol. xxxii, p. 499, 1907), I have no doubt that 

 he is right in placing it in the family Cerithiidae, and also in 

 including in Fischer's genus several forms formerly called Cerithhim 

 and Bittium. 



In mj^ opinion the names Cerithium eludens, Bayle, and duhium, 

 G. B. Sow., are synonyms of Plesiotrochus monachus, Crosse. This 

 much less trochiform species forms an interesting link between 

 Cerithium and the typical Plesiotrochus. 



1. Plesiotrochus exilis (Pease). 



Trochus exilis, Pease, Amer. Journ. Conch., vol. iii, p. 286, 



pi. xxiv, fig. 7, 1867. 

 Plesiotrochus souverhieamis, Fischer, J. de Concli., vol. xxvi, 



p. 212, 1878. 



2. Plesiotrochus impendens (Hedley). 



Cerithium imfendens, Hedlev, Mem. Aust. Mus., Mem. iii, 

 pt. vii, p. 434, fig. 23, 1899. 



3. Plesiotrochus pagodiformis, Hedley, Proc. Linn. Soc. N.S.W., 



vol. xxxii, p. 498, pi. xvii, fig. 16, 1907. 



4. Plesiotrochus monachus (Crosse & Fischer). 



Cerithium monachus, Crosse & Fisch., J. de Conch., vol. xii, 

 p. 347, 1864 ; vol. xiii, p. 45, pi. iii, figs. 17, 18, 1865. 



C. dubium, Sow., Thes. Conch., vol. ii, p. 864, pi. clxxxi, 

 fig. 120, 1855 {non Sow., Min. Conch., vol. ii, p. 108, 1816). 



C. eludens, Bayle, J. de Conch., 1880, p. 245. 



