MONSTROSITIES — SCALARIFORMITY. 
117 
It was originally described in 1818 as a new species by Capt. Brown 
under the name of Helix cochlea, and Dr. Tnrton the following year 
described it as Helix terehra, but it was afterwards reduced from specific 
rank and the shell allocated to its appropriate species when its affinities 
were recognized. This dislocated spiral coiling seems to be more 
especially the result of the peculiar and abnormal conditions arising 
from living in waters modified by the warm water, and perhaps other 
substances, emanating from steam engines. A reservoir at Swansea, 
the water of which was kept at a high temperature by the influx of 
condensed steam, etc., from steam engines, was inhabited by 
Planorhis marginatus, and all the specimens tended to assume the 
raised spiral form. The mill-pond at Roch- 
dale, which has been so often recorded for 
its plentifnl production of this monstrosity, 
was subject to the same conditions as the 
reservoir at Swansea. The whorls may also 
become (piite detached and separate like 
a Verrnetus or a corkscrew, although the 
extreme openness of the spiral coiling is 
seldom so pronounced and regular as in the paheogeuic specimen 
of Planorhis spirorbis I have figured. Sometimes, but more rarely, 
the whorls coil closely in a cylindrical 
fashion, like those of Pupa ; I have only 
seen this deviation in Planorhis mar- 
ginatus and Planorhis vortex. Planorhis 
marginatus seems especially liable to 
produce these scalarid monstrosities, but 
Planorhis spirorbis shows the most pro- 
nounced and decided tendency to revert to the original sinistral 
direction of coiling. 
One of the causes inducing this scalarity in the Planorhes has 
been conclusively demonstrated by Van den Broeck and others to be 
the thick and occasionally matted growth of Lemna minor covering 
the ponds inhabited by the Planoi'hes, and preventing the easy access 
of the discoidal shells to the surface for respiration, the young and 
the scalariform specimens being proved by experiment to penetrate 
the dense vegetation with far greater ease and celerity. 
In the Lake of Magnbe, in Belgium, where this deviation in 
Planorhis marginatus was found in great abundance, and the normal 
Fig. 264. — Planorhis vortex 
monst. colu77iclla Taylor X 3, 
Ditch, East Moors, Cardiff, 
Collected by Mr. F. W. Wotton. 
Fig. 'l^.-Pla7iorbis spirorbis 
monst. priscu77i Taylor X 3, 
Pond, Hayling Island, Hants., 
Collected by Mr. H. H. Haines. 
