15 
temporary convenience, and it was on account of a failure to distin- 
guish between names of this category and names proposed for sup- 
posed genera that the exception was rescinded. In 1907. at the Boston 
congress, a helminthologist proposed the following, which was 
adopted as part of the code : 
“ Certain biological groups which have been proposed distinctly 
as collective groups, not as systematic units, may be treated for con- 
venience as if they were genera, but they require no type species. Ex- 
amples: Agamodistomum, Amphistomulum^ Agamojilaria^ Againo- 
mermis^ SparganumA 
As will be shown below, Plerocercus and Plerocercoides also now 
come under this paragraph. As matters now stand it is necessary to 
show that a name was distinctly proposed to designate an artificial 
collective gi’oup in order to bring it under this provision, A name 
like Cysticercus or Echinococcus, originally proposed as generic, not 
distinctly to cover an admittedly artificial group, is subject, noAv as 
before, to the law of priority. 
The term PJerocercu^ full; KipKo^, tail) was proposed by 
Braun (1883a, 98) as designation for the parenchymatous cysticerci 
(namely those the caudal portion of Avhich contains no fluid) as dis- 
tinguished from the bladder worms, or true cysticerci : thus it is a de- 
scriptive term for a stage of deA^lopment (laiwa) possessing certain 
characters, but not the designation, of a systematic unit. As ex- 
amples, Braun cited a Plerocercus \Ditliyridium lacertce Vah] of 
lizards and a Plerocercus of Tetrarhynchus. This plerocercus of the 
lizards happens to figure in a true nomenclatural sense, nameh" in 
the genus — 
DITHYRIDIUM RudolpM, 1819. 
1819: Dithyridium Rud., 1819a, 559 {lacerta; viridis, lacertx muralis; Europe. 
Type by later absolute tautonymy Piestocystis dithyridium — DitJi. la- 
certce). 
1850; Piestocystis Dies., 1850a, 178, 494-496 {Dithyridium 1819, renamed ; in- 
cludes as valid species P. crispa (Rud.), P. rugosa Dies., P. variahilis 
Dies., P. dithyridium Dies., [for Dithyridium of Rud., namely D. 
Zaccrfa’ Valenciennes, 1844]). 
This genus is based upon a larval tapeworm (a plerocercus) pro- 
vided with four suckers, and is classified in the family Taeniidse; the 
type species occurs in Europe in lizards of the genus Lacerta. 
In 1866 Baillet mentioned a parenchymatous cysticercus (namely 
a plerocercus) from the abdominal cavity of the cat and the dog. In 
1882 or 1883. Blumberg described this form as a new species under 
the name Cysticercus elongatus; the latter name however was already 
preoccupied (cf. C. elongatus Leuck., 1812). In 1885. Railliet re- 
named this form C ysticercus hailleti, and in 1893a, p. 311, he classi- 
24800— Bull. 40—08—2 
