T42 



ON THE OCCURRENCE OF UNIO SINUATUS Lam. 

 IN THE BRITISH ISLES. 



By J. WILFRID JACKSON, F.G.S. 



(Read before the Society, Nov. gth, igio). 



For some considerable time past I have been accumulating material 

 for a paper on the distribution of the Pearl Mussel {Margaritaua 

 margaritifera L.) in the British Isles, and in the course of my work 

 I have seen and examined specimens from some thirty or more dis- 

 tinct localities in the United Kingdom, as well as from several places 

 on the continent and North America. This has, naturally, necessi- 

 tated access to several individual collections, the most important being 

 that of Mr. R. Standen, who possesses perhaps the finest series in the 

 country. In working through this latter collection, paying particular 

 attention to the hinge-teeth, I was somewhat surprised to come across 

 a single specimen labelled : " R. Clouden, Dumfries (1865), v. sinuata 

 (Peace Collection)," which possesses well-developed lateral teeth, as 

 well as very strong pseudocardinals. This specimen, Mr. Standen 

 tells me, was presented to him by the late Mr. R. D. Darbishire, along 

 with others from various localities, all of which formed part of the 

 " Thos. Peace Collection," acquired some twenty years ago by Mr. 

 Darbishire. The shell is in good preservation, hardly eroded at the 

 umbones, and measures antero-posteriorly 112 mm., dorso-ventrally 

 62 mm., laterally 34 mm. The habitat, R. Clouden, or Cluden 

 Water, a rivulet in South-west Dumfriesshire and a tributary of the 

 R. Nith, is mentioned by Rimmer in his Land and Freshwater Shells 

 of the British Isles, London, 1880, p. 16, as a locality for var. diniata 

 of the Pearl Mussel. 



Although I examined carefully the hinges of scores of others, many 

 of which have been recorded as var. sirmata Lam., I failed to dis- 

 cover any more which possess these lateral lamellcC. This feature, as 

 is well known to most conchologists, is characteristic of the genus 

 U7iio^ but is quite absent in Margaritana, to which genus the true 

 pearl mussel belongs. It is, therefore, quite evident that there has 

 been some want of observation in the past as to this point, collectors 

 simply relying on the outward appearance of their shells, without 

 examining the hinge, and have thus wrongly applied Lamarck's specific 

 name sinuata to shells oi M. margaritifera L., which possess an indented 

 or sinuate lower margin. A new name will, therefore, be required for 

 this form in place of the so-called var. sinuata (Auct.), which should 

 be struck out of the Society's List in order to prevent further 

 confusion. As far as I can ascertain, this indented form of Af. 



