DONKIN, IN REPLY TO WALKER-ARNOTT. a 
vation, that such statements are erroneous. Now, as he has 
never examined a single specimen of Pl. carinatum, his ob- 
jections to this form are simply imaginary, and being, there- 
fore, of no importance, according to the principles which 
regulate the determination of every scientific truth, they are 
unworthy of refutation. It may not be amiss here to repeat, 
even emphatically, that my published description of this 
remarkable species is perfectly correct, both as regards the 
shape of its S.V. and F.V. and as regards the nature of the 
strie, of which there are two sets obliquely arranged, and 
easily resolvable, with sufficient power and proper illumina- 
tion, into cellules, having a quincuncial arrangement; but 
owing to the valve being compressed laterally towards the 
median line into a keel, these markings are most easily seen 
on the F.V. 
Dr. Arnott pronounces my Pl. arcuatum to be Pl. fasciola. 
In reply, I must observe that whereas the former is a strictly 
marine form, being only found on the open shore or in deep 
“water, the latter only occurs in brackish water, in the living 
state. But independent of this very important fact, the two 
forms present structural differences of a specific nature, which 
cannot be ignored. In the first place, the extremities of Pl. 
arcuatum have each a strong double curve, that is, each is 
strongly sigmoid between its base and its apex. Whereas 
each extremity of Pl. fasciola has only a single curve, or in 
other words, is gently arcuate between its base and apex. In 
the second place, the extremities of Pl. arcuatum are much 
longer than those of Pl. fasciola. In my figure of the former 
species the extremities are represented much too short, a fact 
kindly pointed out to me by Dr. Greville. Thirdly, the striz 
are much finer than those of Pl. fasciola. 
As to my Apr. duplex being the same form as Apr. palu- 
dosa, as alleged by Dr. Arnott, I must observe that the very 
fact of the former being a strictly marine species, while the 
latter, according to the late Professor Smith, is the only 
fresh-water form in the whole genus, renders such an allega- 
tion simply untenable. Besides, a mere examination of 
figures of the two forms cannot fail to convince any one of a 
specific difference. 
Dr. Arnott says my figures and descriptions lead him to 
believe that my Pl. Wausbeckii and minutum, and probably also 
angustum, are the same form as Pl. rectum, or, in other words, 
his Apr. Ralfsii. I am, however, led to suspect that his 
examinations of my descriptions, at least, must have been 
exceedingly superficial, otherwise he could not have arrived 
at such a conclusion. Dr. Arnott also states his belief that 
