74 NELSON AND TAYLOR ! ON YORKSHIRE MOLLUSCA. 



great profusion and in a variety of species. The Limnaese are 

 world-wide in distribution, though they would seem to be more 

 numerous in the temperate regions. 



This group is especially liable to erosion, and to loss of 

 the apex or decollation, and many causes may, according to 

 those who have studied the subject, tend to bring about these 

 results. 



They are stated by Eisig to be proterandrous, the male 

 organs developing usually in advance of those of the other sex. 



Amongst other parasites, they are liable to be infested with 

 Tetracotyle typica Dies., otherwise T. lymncei Pagenstecher; but 

 the most important parasite of this group is undoubtedly the 

 Distovia hepaticmn, found mostly upon L. truncatula, and 

 which is the cause of rot in sheep. Several species of Gordius 

 i^G. aqiiaticus and G. gratianopolensis) have also been recorded 

 as infesting this genus. 



Sub-Genus AMPHIPEPLEA Nilsson. 



.Shell extremely thin, fragile, and glossy, with a very short spire. 

 Mantle lobes very extensile and capable of entirely enveloping the shell. 



This group is, in our opinion, sufficiently distinct to be en- 

 titled to full generic rank. We, however, refrain from carrying 

 out this opinion here, from a desire to continue to conform to 

 the arrangement adopted by Dr. Jeffreys. 



Limnsea glutinosa Miill. 



Extremely local. 



This interesting species does not appear to have been 

 found in Yorkshire since the late Mr. Bean ceased to find it at 

 Scarborough, until its recent discovery in immense numbers in 

 the Skidby drain, near Hull, where it was first found in October, 

 1889, by Mr. James F. Robinson, President of the Hull 

 Scientific Club. Its former habitat at Scarborough is described 



Trans. Y.N.U., iS9o(pub. 1891). Series C 



