26 THE SCOTTISH NATURALIST 



Scottish forms of L. peregra in greater detail witli reference to tlie 

 anatomy. In the hill-torrents of Ulva a third form occurs, very like 

 lacustris but still smaller and with the spire, when it is not com- 

 pletely eroded, more exserted. The body-whorl is also narrower. 

 Finally, in a stream running along the high road on Mull, just 

 opposite Ulva, and dammed at one point to form a small duck- 

 pond, a fourth form is common, with larger and broader shells than 

 lacustris^ but still not more than half the size of specimens from 

 really favourable localities. We did not find the form called /zV/'« 

 by Jeffreys and originally described from Ulva. 



Limncea tniHcatula. — This species was found with L. peregra on 

 Ulva and Innis Kenneth, but in very much smaller numbers. It 

 was also found in a small pool in a little cave in the cliffs of Eorsa. 

 The shells were small but otherwise typical. Its occurrence on those 

 islands is of practical importance, as all the older sheep on Ulva are 

 infected by the liver-fluke {Fasciola hepatica), for which it acts as 

 the intermediate host. Mr A. J. Hesse, who found that a very large 

 proportion of the individuals of L. peregra from Ulva were infected 

 with the cercarice of other trematodes, did not find any cercarins in 

 the few individuals of L. truncatula from that island he was able to 

 examine in August. 



Hydrohia (or Paludestrind) tdvce. — Living examples of this 

 species are abundant in the little pools of a salt marsh formed by 

 the delta of one of the streams on Ulva. These pools had a muddy 

 bottom, and there was in them a complete absence of the alga from 

 which {iiot from the island) the mollusc gains its specific name. In 

 spring tides the pools were covered by the sea. Their fauna con- 

 sisted mainly of small amphipods {Coryphwjn), of various species of 

 Liftorina, of young Carcinus mcenas and eels, and of H. uIvcb. The 

 shells of the last were much corroded and often distorted. On the 

 shore of a small bay forar:ied by a landing stage on the shore about 

 a quarter of a mile from the marsh we found, adhering to the lower 

 surface of stones between tide-marks, many living specimens of the 

 form figured by Jeffreys (PI. Ixix. fig. 2) as " Rissoa uIvcb var. Barkei.^' 

 The animal of this form closely resembles that of the typical 7ilvce so 

 far as external features are concerned. Mr Robson, who has been kind 

 enough to examine specimens with me, agrees that the shells have the 

 appearance of being specifically distinct, but that only a detailed ana- 

 tomical investigation would afford accurate data for a determination. 



Pisidium personatum. — Not uncommon in a very thin-shelled 

 depauperate form in the hill-streams of Ulva. In the springs on 

 Innis Kenneth shells are also small but much thicker. Our 

 specimens were identified by Messrs Stelfox, Phillips, and Oldham. 



