Miscellaneous, 19 



C. Rolpkii, collected by him at Charlton near Woolwich, it presents 

 the following differences. 



In C. Rolphii the shell is more elongate and the spire more gradually 

 decreasing in breadth towards the apex, notventricose, and suddenly 

 narrowing to the attenuated upper portion of the spire, as in C. Mor- 

 filleti. It is lighter in colour, with a fulvous tint, rather than the 

 purphsh hue which pervades the specimens of the latter ; the basal 

 crest is not prominent or sharply defined, and the rima is narrow, 

 and elongated nearly to the base ; whereas in C. Mortilleti the crest 

 forms a strong funiculate keel, and the periomphalus is open and 

 semicircular. In C. Rolphii the lower lamella is cruciate ; both 

 species are deficient in the palatal callus so conspicuous in the true 

 C. plicatula, Dr. 



Length of C. Rolphii, 14 mill. 



r of C. Mortilleti from Charlton-Kings near Cheltenham, 



14 mill. .^^^ 

 of ditto from Charing, 11 mill. 



C. Rolphii has 1 0|- whorls ; C. Mortilleti only 9| in English ex- 

 amples, but a specimen of a more slender variety, which 1 got at 

 Chaud-fontaine in Belgium, exhibits the same number as C. Rolphii. 



Thus the two distant counties of Kent and Gloucestershire produce 

 a shell which has so long been unaccountably overlooked on the 

 continent, as well as in England. There are some who still persist 

 in confounding C. Rolphii with plicatula of Draparnaud, notwith- 

 standing the differences observable, and the assurance of De Ferussac, 

 as reported by Gray, Independently of other characters the more 

 remote costation oi plicatula, its palatal callus, and different mode of 

 rimation sufficiently distinguish it. In colour its ranges with C. Mor- 

 tilleti, the differences of which were pointed out by Adolf Schmidt 

 in the * Annals ' for January last. 



C. plicatula, omitted in Mr. Jeffreys' s notes on the Swiss Mol- 

 lusca (x\nnals for January 1855), but noticed in his collection cata- 

 logue, occurs at Monthey and St. Maurice in the Valais, as well as at 

 Glarus. In both catalogues he has omitted C. pumila, Ziegler, 

 var. /3, Pfr., and C. lineolafa. Held. The latter shell I got in the 

 tract explored by him, between Chillon and Villeneuve, as well as in 

 the north of Switzerland. 



W. H. Benson. 



12th June 1856. 



On the Siliceous Sporangial Sheath of the Diatomacese. 



In the 16th volume (1855, p. 92) of the * Annals of Natural Hi- 

 story,' I pointed out the occurrence of a siliceous sheath enveloping 

 the sporangial frustule of a Navicula {Amphirhynchusl), and stated 

 that " it was composed of silex, i. e. was indestructible by heat and 

 nitric acid ;" also, that it was "colourless, elongate, rounded at the 

 ends, and furnished with coarse transverse strise, or depressions, 

 through which the hne of fracture runs when the object is crushed." 



