296 DISCOVERY REPORTS 



often in considerable numbers, but always as an inconspicuous item in the fauna. But at 

 station WS 76, where the fauna, though varied, was not rich, it appears as a dominant 

 form. It does not occur at all at the nearest adjacent stations 51 and WS 73 which are in 

 shallower water, and presumably the somewhat greater depth favoured its unusual 

 development. But at present we know very little of the causes favouring the abnormal 

 increase of a species in one locality as compared with another to all appearances 

 identical. 



Apart from the interest raised by the enormous development of these dominant forms, 

 the Falkland fauna is itself full of interest. It includes many species of world-wide 

 distribution as a matter of course, and many other species known from similar cold 

 temperate waters in the northern hemisphere. As the Falkland Islands lie in approxi- 

 mately the same latitude as southern England it is not surprising that many typical 

 British species figure in our lists. But it is rather a matter for wonder that some of the 

 rarer British species (e.g. Lagena millettii, Lingidina quadrata, L. tramliicida) and others 

 which we have known from British waters for many years but had not yet had an 

 opportunity of describing (e.g. Liugulina falcota, Potellinoides conica, P. depressa) 

 should also be found in such a distant locality. Considering their extreme rarity in each 

 of these widely separated areas, it remains an insoluble problem how these species 

 achieved their distribution across the deep water of the Atlantic, for their migration by 

 the shore line would almost certainly be prevented by differences of temperature. 



Yet in spite of general resemblances to the British fauna, there are certain species 

 found in both areas which present differences in the Falklands suggesting a distinct local 

 race, the variations seldom being sufficient in our opinion to justify varietal, much less 

 specific distinction. Lagena zvilliamsom (Alcock) is one of the commonest British 

 Lagenae. The type is a pyriform costate shell with 12-18 costae and hexagonal ornament 

 on a collar round the neck. The Falkland form described under the name L. vilarde- 

 boana (d'Orbigny) is almost equally common in our area where we recorded it from 

 eighteen stations. The costae are much more numerous but are weakly developed, and 

 the hexagonal ornament is reduced to a series of pin-pricks on the collar. Discorbis iiitida 

 (Williamson), again, is a typical and fairly common British species. The Falkland Islands 

 specimens are more concave on the ventral side, with an increased convexity of the dorsal 

 side, generally more robust and less '' nitida'' than the type. 



But in neither case should we have any hesitation in assigning the two races to the 

 same species, and the differences are no doubt due to long isolation under somewhat 

 varied conditions. Many similar cases could be quoted. 



The Falklands possess several species which are either wholly or almost wholly pe- 

 culiar to the area. Notable among these is Ehrenbergina {CassidHlina)pupa, first described 

 by d'Orbigny in 1839 from these islands. He states that it is much rarer than Cassididim 

 crassa, but we record it from twenty-eight stations, and at some of them it is almost as 

 common as that species. Outside the Falkland Islands it may be said to be almost 

 unknown. The only records which can be accepted, even with suspicion, are from the 

 west coast of Patagonia and from Rio de la Plata. Nor is it replaced in abundance 



