276 



ON THE OCCURRENCE OF CLAUSILIA DUBIA Drap. 

 IN KENT. 



By II. OVERTON. 



(Read before the Society. Sept. 13, iQii). 



During the autumn of 1897. whilst collecting in the di.stiict of Dover, 

 I discovered a flourishing colony of Claitsi/ia on and near one of tlie 

 walls of the castle, and on reaching home these specimens were put 

 on one side as being merely fine examples of C. l>identata Strom.- 

 Since that time, specimens from many localities have passed througlv 

 my hands, and I have also been afforded the opportunity of examin- 

 ing numerous series belonging to other collectors. 



The specimens in question may be divided into two forms : one 

 long and fairly stout in proportion to the length (average alt., 

 i4'39 mm. ; average diam., 3*27 mm.); and the other short and of 

 the same diameter, giving the shell a more tumid appearance (aver- 

 age alt., i2"53 mm. ; average diam., 3'27 mm.). 



On comparing these latter with continental forms of C. dubia Drap., 

 I fail to discover any difference whatever, and have no hesitation in 

 stating them to be this species, whilst on comparing the other form 

 with C. cravenensis Taylor from Westmorland, the shells appear to 

 agree in every detail. 



Since these shells were all collected from one spot, and it is a fact 

 that a complete series can be formed linking up the two extremes, 

 it must obviously follow that C. cravenensis Taylor is identical with 

 C. dubia Drap. I may state that this opinion is also shared by 

 Mr. A. S. Kennard, F.G.S., to whom I am indebted for some of the 

 Continental forms with which my specimens were compared. 



That the shells in question are C. dubia Drap. there is not the 

 slightest doubt. The lamellae are identical, and whilst in the Dover 

 specimens the rug?e appear to have faint dots or ridges running 

 parallel with the suture, giving the shell a similar appearance in 

 texture to C. bidentata Strom, this feature can also be- detected in 

 C. dubia Drap. 



It is worthy of note that many continental authors have considered 

 C. dubia Drap. to be a British species and there can be no doubt that 

 this view is correct. 



