BY C. HEDLEY. 499 



ceptible below the suture, but swell towards the periphery and 

 undulate the upper limb of the groove where they terminate. 

 The whole surface is roughened by sharp, close, minute spiral 

 threads, numbering about 16 on the penultimate; their interstices 

 are latticed by radial striae. Protoconch smooth, two-whorled, 

 turbinate. Aperture unfinished rhomboidal, grooved within by 

 the imprint of the spiral threads. Columella straight, its edge 

 a trifle re|lected. Canal very short and broad. Height 6 mm.; 

 breadth 3*5 mm. 



Abundant in 17-20 fathoms off Mast Head. One specimen 

 was taken by Mr. A. U. Henn off Cape Sidmouth, Queensland, 

 and 1 found it plentiful round the Hope Islands, in 5-10 fathoms. 



The genus Plesiotrochus seems so little known to conchologists 

 that a few remarks on it may be acceptable. It was formed by 

 P. Fischer* for the reception of his new P. souverbianus, from 

 the Loyalty Islands. The classification of the genus puzzled him; 

 a resemblance to the Cerithiidse caught his eye, an affinity to the 

 Littorinidai was mentioned, but tinally, and with a mark of 

 interrogation, he placed it in Planaxidse. In the opinion of the 

 majority, the genus is best suited with the Cerithiidse. 



It was suggested by Pilsbryf that Trochus exilis Pease, | from 

 the Paumotus, should he conveyed to Plesiotrochus. After 

 examining specimens from the Paumotus, the Ellice and the 

 Loyalty Islands, I would unite P. souverbianus to P. exilis as an 

 absolute synonym. To the same genus I now refer my Cerithium 

 impeudens^ from Funafuti. I would further enlarge Plesiotrochus 

 by the addition of the Tasmanian Cerithium monachus Crosse & 

 Fischer. 1 1 The resemblance of the latter to Plesiotrochus has been 

 remarked by Melvill & Standen.^ Angas referred C. monaclius 

 to Potainides*'^ Watson's comment on his Bittium oosimense'W 



* Fischer, Journ. de Conch, xxvi. 1878, p. 212. 



t Pilsbry, Man. Conch, xi. 1889, p. 490. 



X Pease, Am. Journ. Conch. 1867, p 286, pl.24, fig. 7. 



§ Hedley, Mem. Austr. Mus. iii. 1899, p.434, fig.23. 



Ij Crosse & Fischer, Journ. de Conch. 1864, p. 347. 



IF Melvill & Standen, Journ. of Conch, viii. 1897, p. 409. 



** Angas, Proc. Zool. Soc. 1865, p. 171- 



+t Watson, Chall. Rep. Zool. xv. 1886, p.ooO. 



