EDWARDS : UNIO TUMIDUS IN LEICESTERSHIRE. I23 



shells from this locality were first identified as var. ponderosa Pascal, by Mr. H. E. 

 Quiiter, in a local list published in the Transactions of the Leicester Lit. and Phil. 

 Society, part vii. , April, 1888. This record is, I understand, the first for this coun- 

 try of this fine variety. I have myself taken living specimens at different times since 

 1893 ii^ this locality, but last August when I visited the pond I could with diffi- 

 culty obtain living examples, although dead shells abounded. I handled perhaps 

 two hundred shells, but only found ten living ones. Many were evidently quite 

 recently defunct, since the remains of the animals were still in their shells. The 

 place had become so choked with decaying branches and leaves from the surround- 

 ing trees that dredging was impracticable, and I had to feel for the shells amongst 

 the mud and refuse, and thus was only able to work round the edge of the pond. 

 I found that the causes so fatal to the Unio tuinidus were having an equally bane- 

 ful effect upon the Anodonta cygncea, of which scarcely a living example remained, 

 and also upon Limntsa stagnalis, Planorbis corneus^ and other species, of which I 

 could only find dead shells. The Anodons in this pond are of the usual thin type, 

 and not at all incrassate, which seems somewhat remarkable considering the heavy 

 and ponderous growth attained by the Unios. Only one specimen of Unio 

 pictoniin, and that dead, was found on this visit. On October 25th I again visited 

 the pond, but failed to obtain a living Unio, although I did find two living 

 Anodons, so that both species, so far as one can judge from the shells obtainable 

 at the edge of the pond, appear to be fast dying out. Whether there are some still 

 living in the middle of the pond I cannot say, but judging from the condition of 

 the water it is highly improbable, for it is so highly charged with sulphuretted 

 hydrogen that a silver ring I wore was soon turned to a brassy black colour. The 

 last two summers have been very dry here, and the pond having only one small 

 inlet has received but little fresh water for two seasons. The formation upon which 

 the pond is situated is Lower Lias Clay with a surface of Boulder Clay. When Mr. 

 Quiiter worked it in 1887-8 he found that the water contained 15*2 grains of car- 

 bonate of lime per gallon. It is rather curious that whilst Wistow Pond has only 

 produced two specimens of U. pictorinn, Saddington Reservoir three or four miles 

 away has this species in plenty, but has only produced one valve of U, tinnidus, 

 although I examined nearly the whole area of the bed when it was practically 

 empty last year. The Wistow specimens of U. tumidus are unusually large and 

 massive, and very dark, practically black, in colour. My largest measures 120 mm. 

 by 66 mm., and weighs 5 "5 oz. It would thus seem that they have increased in 

 dimensions and weight since Norman's time, assuming that he recorded the largest, 

 for his weights and measurements would answer to the smallest I have taken. The 

 shells are mostly a little eroded at the umbonal region. On the occasion of the 

 Annual Meeting of the Society, in October last, I was pleased to have an oppor- 

 tunity of comparing my Wistow specimens with a remarkably fine series of Unio 

 tumidns and U. pictoruni obtained under very similar conditions from a pond at 

 Ward End near Birmingham by Mr. R. D. Darbishire, and now in the Manchester 

 Museum. So much do the U. tumidus in this series resemble mine in form, colour 

 and general characteristics, that were they mixed it would be almost impossible to 

 separate them. But the largest of the Birmingham specimens far surpass the 

 Leicester ones both in dimensions and weight, the biggest measuring 130 mm. by 

 67 mm., and weighing 7*5 oz. The series — a large drawerful — is further remark- 

 able also in that an apparent transition in form from U. tuinidus to U. pictomtn 

 is most strikingly demonstrated, which certainly gives one food for reflection and 

 raises doubts as to the stability of the two forms — as species I — Thos. Edwards 

 {Read befo7-e the Society, November 14th, 1906). 



