WALKER-ARNOTT, ON ARACHNOIDISCUS. 165 
pearance. Dr. Donkin will allow me also to add, that his 
S. V. and F. V. of Pleur. lanceolatum, belong not only to dif- 
ferent species, but to distinct genera; that his Plewr. arcuatum 
is only Pleur. fasciola: his Toxonidea insignis, the well- 
known distorted state of Pleur. estuart ; his Tox. Gregoriana 
the same of either Pleur. strigosum, or angulatum (I have 
seen both distorted), but which I cannot say from his imper- 
fect diagnosis of the species. His Cocconeis excentrica was 
discovered by De Brébisson, in 1852, and was then called by 
him C. orbicularis ; and his Epithemia marina, the HE, Radula 
of the same French gentleman, afterwards distributed in slides 
by Professor Smith as Nitzschia Radula ; this I have long had 
from the Clyde, and also Teignmouth. His Amphiprora 
duplex, judging from the figure and diagnosis, scarcely differs 
from A. paludosa. It is to be regretted that Dr. Donkin did 
not make himself acquamted with what others are doimg, 
before rushing into type; and that, stead of giving the long 
descriptions and figures, and multiplying species unneces- 
sarily, he had limited his species by a short, concise character, 
as every ¢rue naturalist must do who wishes his species to be 
adopted or considered by others. 
