Mr. J. Alder on JefFreysia and Chemnitzia. 461 



observed. The species that I have been able to examine are 

 C. rufescens, O. acuta, 0. Rissoides, O. Eulimoides and O. cylin- 

 drica; in all of which the opercula are similar to what I have 

 represented in my figure, varying a little in the indentation of 

 the margin opposite the longitudinally impressed line*, and the 

 consequent greater prominence of the lower portion of that side, 

 which may possibly be the part Mr. Clark calls the apophysis ; 

 at least it appears to be the portion alluded to by Mr. Gray 

 under the name of the flap ; but it is in the same plane, and con- 

 tinuous in strise with the rest of the operculum. There is a 

 little difference in the fineness of the striae in some of the species, 

 or in the degree of development of the apical turn, but all have 

 the striae in the same direction, and preserve unchanged the 

 typical subspiral character. The instance produced by Mr. Clark 

 in proof of the operculum's varying in the same species is from 

 the genus Trochus. " I have some Tr-ochi," he says, " with the 

 apparently fine annular striae, and others with radiating lines and 

 as grossly spiral as in Littorina vulgaris." It is difficult to un- 

 derstand the extent of variation this is intended to convey. The 

 operculum of Trochus is multispiral, and the whorls are fre- 

 quently so close as to appear like annulae to a casual observer. 

 Now^ does Mr. Clark mean to say that he has got a Trochus with 

 the operculum really annular {apparently is a vague term when 

 facts are in dispute), and that this is of the same species with 

 others having the operculum decidedly spiral ? A more definite 

 assurance of this is desirable, not merely for the sake of localizing 

 a species, but because it militates against the constancy of nature, 

 upon which physiologists are accustomed to rest their faith as 

 the only ground on which generalizations can be made. Our 

 friend Mr. Gray, too, tells us that the spiral and the annular 

 opercula are formed by a different process, the one requiring a 

 gradual movement on the centre as new matter is deposited, 

 while the other remains fixed. 



With regard to the existence of an apophysis in Chemnitzia, 

 I have broken up several specimens in the endeavour to find it 

 out, but without success. I find in some examples a thickening 

 internally of the apical nucleus, with a tendency to become pro- 

 minent. Can this be what Mr. Clark means? Or is it the 

 produced central area, sometimes slightly reflected upon the 

 pillar, which is alluded to above as the flap of Mr. Gray ? We 

 are not informed exactly of its character or where it is situated. 

 All I can say is that I can find nothing at aU resembling the 

 conspicuous erect process of Jeffreysia : nor does Professor Loven, 

 on whose accuracy I place great faith, describe any such process 



* The very slight indentation observed in 0. Rissoides has been omitted 

 in the woodcut accompanying my last letter by mistake of the engraver. 



