VARIATION AND SPECIFIC CHARACTERS 7 



instance only {Pallenopsis patagonica), ovigerous males were collected from both regions, 

 and here also there is a difference of three months (March and April in Antarctic waters, 

 July in the Magellan District). Antarctic specimens of Toiiystylum pfefferiwere carrying 

 ova from October to December, while an ovigerous male referred to the same species 

 was collected off Gough Island in June. Only a few records from South Temperate 

 and Tropical waters are available and these are for the period May to July. 



VARIATION AND SPECIFIC CHARACTERS 



Dr Caiman (1915, p. 6) divided the Pycnogonida into two categories distinguished 

 as follows. "Certain genera and families present large numbers of minutely separated 

 species, the distinguishing characters of which have at least the appearance of in- 

 constancy ; while other groups are composed of few species easily and sharply defined 

 by characters that are relatively invariable." The first category is by far the larger, and 

 I have found considerable variation also in two genera hitherto regarded as monotypic, 

 namely Pentanymphon and Amtrodeciis. On the other hand the five species of Ammothea 

 described as new each appear to be characterized by several distinctive, sharply defined 

 features, although these also may prove to be liable to considerable variation when 

 more material is available. 



The larger genera belonging to the first category, e.g. Colossendeis, Polletiopsis and 

 especially Nymphon, present, as every student of the group is well aware, many systematic 

 difficulties and problems. Previous writers have sometimes taken considerable pains 

 to provide keys to facilitate the task of specific determination, but no one seems to have 

 undertaken a really comprehensive study of the specific characters. Since each 

 succeeding collection includes some new, or apparently new, forms, the multiplication 

 of species cannot profitably be continued until a thorough revision of each genus is first 

 undertaken. This would necessitate, in addition to a re-examination of all the available 

 types of known species, a study of the amount of variation within each species. 

 Frequently the available number of individual specimens is far too small to admit of this. 

 Some knowledge of the post-larval development would also prove most useful, but at 

 present very little is known even in the more common forms. There is some evidence 

 to show that, for example, the relative proportions of tarsus and propodus (often used 

 to distinguish between closely allied species) differ considerably in young and adult 

 specimens of certain species {Colossendeis scoresbii and Pentanymphon antarcticum). 



The key to the determination of the " Longitarsal " species of the genus Colossendeis 

 (p. 11) is little more than an extended and slightly modified edition of that previously 

 given by Dr Caiman (191 5, pp. lo-ii). That drawn up for the determination of the 

 Antarctic species of the genus Ammothea (p. 95) is almost entirely new; as many of the 

 specific characters as possible have been included in the key, which thus incorporates 

 the results of what may be regarded as a preliminary revision of these species. Most of 

 the forms included in this report are now, it is hoped, adequately described and figured, 

 but many species not represented in the Discovery collections are still in need of revision. 



