SYSTEMATIC AND BIOLOGICAL ACCOUNT 35 



them to Keferstein & Ehlers' (1861) Rhabdoon singular e, but left any decision about their identity with 

 Gegenbaur's Chrysomitra for future research to decide. 



Bigelow (191 1 b) in his review of the genus mentioned that eleven so-called species have been 

 described from the Indo-Pacific. Most recent authors have agreed that there is only one Indo-Pacific 

 species of Velella and that it is distinct from the Atlantic one, but Schneider (1898), and Bigelow 

 & Sears (1937) with whom I am inclined to agree, thought it was the same as the Atlantic one. 

 Bigelow (191 1 b) found no features to differentiate more than one Pacific species, but was not prepared 

 to unite it with an Indian Ocean one, because a critical review of Indian Ocean specimens was not then 

 possible ; but he believed that the separation of Pacific and Atlantic species was justified, the breadth 

 in proportion to the length being greater in Pacific than in Atlantic specimens : in other respects he 

 could find no constant difference. 



Bigelow gave measurements of sixteen shells from the Atlantic and of the same number from the 

 Pacific. The ratio of breadth to length of the Atlantic ones was from 32 to 37%, and of the Pacific 

 ones 44 to 51%. For Mediterranean specimens, more than 8 mm. in length, Bigelow & Sears gave 

 a mean ratio of 38-5 %■ It is convenient to call the long axis of a specimen of Velella the 'N./S.' one, 

 and a specimen in which the sail runs ' N.W./S.E.' is called a ' N.W.' one and the type in which it runs 

 'N.E./S.W.' a'S.W.' one. 



An asterisk indicates that the measurement is an estimate of an imperfect specimen. 



5-2 



