1 66 



OBSERVATIONS ON ABNORMAL SPECIMENS OF 



PLANORBIS SPIRORBIS AND OTHER FRESHWATER 



SHELLS AT TENBY. 



By A. G. STUBBS. 



(With Plate IV.) 



(Read before the Society, March 9, iS 



A large series of various abnormal shells was exhibited at the 

 February meeting of the Conchological Society. Since then I have 

 obtained a still larger number from the same place, many of them 

 being truly remarkable. They were collected from a ditch, near the 

 town, which abounds with several species of freshwater molluscs, 

 of which the following is a full list together with some of the chief 

 varieties : — 



Planorbis nautileus and var. crista. 



P. spirorbis and var. albida (one specimen), and many mon- 

 strosities. 



Bullinus hypnorum and var. major, and a few monstrosities. 



Physa fontinalis, typical form, and monstrosities. 



Limnaea peregra, two distinct forms, one abnormal. 



L. palustris vars. elongata and albida. — One fine specimen of 

 the latter variety last year, and two others more recently. Mr. Charles 

 Jefferys, of Tenby, also found another half-grown specimen, on February 

 1 6th of this year. Mr. Standen informs me that only two specimens 

 of the albino form of L. palustris have previously been recorded : — 

 one, from near Leeds, by Mr. J. W. Taylor 1 ; the other from Southport, 

 by Mr. Edward Collier. 2 Monstrosities are frequent. 



Valvata cristata, Sphaerium corneum, and Pisidium pu- 

 sillum are common. 



The following is an account of the principal monstrosities observed: 



Planorbis spirorbis. — Of the abnormal examples of this species, a 

 representative series is shown on Plate IV. They are confined to quite 

 a small reach of the ditch (about sixty yards), though the species is very 

 abundant throughout the whole length, and the one hundred specimens 

 shown only represent a portion of those I have taken. 



The most interesting forms of distortion noted in Planorbis spir- 

 orbis are the following: — 



1. — Almost the entire shell scalariform, like a pile of draughts 

 placed one on top of the other. 



2. — The apical whorls scalariform, the last two or three normal, giv- 

 ing the shell the appearance of a Welshwoman's hat. 



t /. Conch., vol. 1, p. 29. 

 2 Tom. citi) p. 139. 



