6 DONKIN, IN REPLY TO WALKER-ARNOTY. 
and his subsequent foot-note, p. 164, it is clearly apparent, 
that he supposes this form to be an Amphiprora because the 
valve is carinated—the carina being constricted in the middle 
—and the striz transverse. Now, on the other hand, I have 
shown* that the structure of the valve (there being two sets 
of strie, one long., the other ¢rans.), together with its peculiar 
outline and sigmoid median line, and also the absence of 
ale, or lateral plates, as seen in the F.V., prove that it is 
a true Pleurosigma. I have likewise pointed out,t+ that a 
carinated and constricted F.V. is not, when taken in the 
abstract, a generic character, and ought not longer to be relied 
upon as such. This assertion is amply proved by Pl. cari- 
natum { (a genuine Pleurosigma, with oblique stri, easily 
resolvable in the F.V.), which has a very strongly keeled and 
constricted valve. After a careful and frequently repeated 
examination of all our genuine British species, I am con- 
vinced that the presence of ale,$ attached laterally to the 
valve, constitute the only true generic feature of the Amphi- 
prore, while the carinated and constricted F.V., though 
always, more or less, present, is merely of secondary import- 
ance, being possessed by several species of Pleurosigmata with 
straight valves ; thus proving a close natural affinity between 
the two genera. Therefore, as Dr. Arnott admits the absence 
of ale in Pl. rectum (alias Apr. Ralfsii), his attempt to refer 
it to the genus Amphiprora is contrary to analogy, and simply 
a violation of the law, which has been observed by all compe- 
tent observers, in placing new species in this latter genus. 
With regard to the markings of the Amphiprore, I may fur- 
ther add, that when striz are present there is only a single 
set, transversely arranged. This peculiarity is of consider- 
able importance in distinguishing them from their allies, the 
Pleurosigmata. 
Dr. Arnott, however, will probably object to these conclu- 
sions, having taken upon himself to ignore the existence of 
any such species as Pl. carinatum ; he denies the fact of its 
having oblique striz, and observes: “I do not believe the 
stri are oblique, but only appear so in consequence of the 
osition of the light.”? Now, I wish to know on what grounds 
r. Arnott considers himself justified in contradicting the 
statements of others on any scientific subject, without 
having in the first instance satisfied himself, by actual obser- 
* See my description of Pd. rectum, n. sp., ‘Trans. Micros. Soe.,’ vol. vi. 
+ In Op. cit., Postscript. 
+ See description of this species, Op. cit. 
§ The universal presence of ale in the genus Amphiprora was first pointed 
out by my late lamented friend, Professor W. Gregory. 
