DESMID FLORA OF DARTMOOR. 253 



the highly developed system of spines of the furcate Staurastra, 

 yet such a desmid as Micrasterias denticulata should be suffi- 

 ciently formidable to secure immunity from attack. Frequent 

 specimens of the larger desmids, more particularly of the genus 

 Euastrum, have occurred from the Dartmoor bogs which were 

 completely enveloped in hyaline cases. Such an one is figured 

 on Plate 18, fig. 6. For some time I looked upon these cases as- 

 being due to a secretion of mucus by the desmid itself, but as 

 the case was obviously a completely separate structure the 

 solution did not appear to be very satisfactory. I was fortunate 

 enough later to come across one of these hyaline cases containing 

 two desmids, a specimen of Cosmarium Brehissonii and a species 

 of Euastrum. It then occurred to me that the cases were 

 probably secreted around the desmid (or desmids) in its passage 

 through the alimentary canal of some aquatic animal owing to 

 its inability to digest it. 



In the field each collection, however unimportant the habitat, 

 was kept separate and duly labelled. At home the various 

 collections were worked out as individual units and the results 

 entered in a field-book, so that it is possible to state exactly what 

 species each station yielded. The various stations were then 

 grouped under their respective districts. The result in species, 

 varieties and forms from individual bogs varies with the extent 

 and apparent age of the bog and shows some degree of con- 

 stancy. That is to say, unimportant stations yielded from 70 

 to 80 records, average-sized bogs 120 to 130, while from the 

 extensive and ancient bogs of the Raybarrow and Merripit type 

 230 records would be obtained. The total number of species, 

 varieties and " forms " entered in my field-book for the district 

 dealt with in this paper is considerably over four hundred ; but 

 as many species require careful re-examination, and as a con- 

 siderable number of the Staurastra cannot at present be satis- 

 factorily named, the total number given in the census table 

 has been reduced to about four hundred. Furthermore, the 

 material cannot yet be considered exhausted, as prolonged 

 examination would certainly result in many additions to the 

 list. 



Zygospores are not infrequent in the Dartmoor gatherings. 



JouRN. Q. M. C, Series II.— No. 80. 19 



