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 A HISTORY OF HERTFORDSHIRE and Mr. Francis Ransom 1 a list of twenty-nine additional species found chiefly in the north of the county. A few years later Mr. Robinson 8 combined these two lists into one and added twenty-six species, increas- ing the number to 153. His list includes eighteen of Hassall's species, leaving three to add, which bring up the total number to 156, as in the following enumeration Epithemia 8 Tryblionella 2 Synedra. 13 Achnanthes. . 3 Euonotia I Cymatopleura 4 Cocconema. 4 Achnanthidium 2 Cymbella 3 Nitzschia 9 Gomphonema 12 Diatoma. 3 Amphora 2 Amphipleura i Meridiem. . I Tabellaria 2 Cocconeis 2 Navicula. 25 Himantidium 3 Melosira. I Cyclotella 3 Pinnularia 1 5 Odontidium. 4 Orthosira 3 Campylodiscus 2 Stauroneis. 5 Denticula 2 Encyonema 2 Surirella. i o Gyrosigma. 4 Fragillaria. 2 Colletonema 3 This list is arranged in accordance with the best work on the British diatoms, and the latest although published nearly half a century ago, Smith's Synopsis of the British Diatomace<% (18536), except that Gyrosigma has been substituted for Pleurosigma, having the priority by eight years. The species admitted are very nearly the same as the 135 enumerated in Pryor's Flora (pp. 51820), with the addition of the species since added by Mr. Robinson. Frustrulia viridis in the Flora is the same diatom as Pinnularia viridis, Cocconema ventricosa of Hassall appears to be C. parvum of Smith, and Bacillaria paradoxa was entered in error, Hassall not having been sure of the locality of his specimen, and saying that the less likely supposition was that it was gathered by him- self in the neighbourhood of Cheshunt. The following appear to be our rarer species : Cyclotella rotu/a, Surirella amphioxys^ Cymatopleura parallela, Nitzscbia vivax, Navicula minu- tu/a, N. tumida, Synedra hamata, Cocconema parvum, and Achnanthes subses- silis. So little is known of the distribution of the diatoms in Britain that some of these species may be more frequent than might be sup- posed from the published records of their occurrence. The rare Achnanthes subsessilis was found by Mr. Robinson 8 in the saucer of a flower-pot in his garden at Hertford. He mounted from this saucer on a slide a single drop of water, which he found to yield upwards of 200,000 separate frustules, and he estimated that these occu- pied only about one-twenty-fifth part of the drop. In further illustra- tion of the minuteness of diatoms he mounted a slide of them with a very small needle (of the size known as No. 10), and was able to show' under the microscope, within the eye of the needle, several hundreds of diatoms of many different species ; and he also mentioned in the paper referred to that if four specimens of one of the smaller Hertford species, Surirella minuta, were placed in a row, the length of that row would equal the thickness of an ordinary sheet of note-paper. Diatoms are easily distinguishable from the rest of the Algas by 1 'Diatoms ; their Nature and Habits,' op. cit. p. 206 (1885). 8 'Observations on Diatomaceae,' op. cit. vol. iv. p. 199 (1887). 3 See his paper in Trans. Herts Nat. Hist. Sac. vol. iii. p. 4. 68