342 VELELLA. 



ground, from a study of specimens "aus den verschiedensten Meeresgebieten," 

 that all Velellas, both Atlantic and Indo-Pacific, belong to but a single species. 



Leaving out of the discussion the early descriptions, none of which are based 

 on sufficiently stable features to be of any real assistance, we find the following 

 characters suggested as points of difference between the Atlantic and Indo- 

 Pacific Velellas. According to A. Agassiz ('65) the numerous specimens which 

 he collected in the Straits of Fuca are much more nearly rectangular in outline 

 than those from the West Indies. Chun ('97b, p. 96) found that "die Form des 

 Mantels (limbus) weicht immerhin so auffallig von jener der V. spirmis ab, 

 dass jedenfalls die pacifische Art von der atlantischen zu trennen ist." Unfor- 

 tunately, however, he gave no details as to exactly what the difference is. 

 Fewkes ('89b) and Lens and \an Riemsdijk (:08) suggest that the Pacific spe- 

 cies is characterized by a triangular crest. , 



I have been able to test the importance of these supposed differences on 

 the considerable series from various parts of the Pacific and Atlantic, listed 

 below, with the result that I believe that the separation of the two species is 

 justified. The difference in form, especially of the shell, suggested in Esch- 

 scholtz's figures ('29, taf. 15), appears to be constant even if slight, the breadth 

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

 in the former the shell is more nearly rectangular with the long sides straighter; 

 in the latter, it is elliptical, with the long sides more curved. 



The exact proportions of a series of sixteen specimens of each species are 

 given in the table below, from which it is also evident that the difference is 

 not only considerable, but discontinuous since all the Pacific examples are broader 

 proportionately than any of the Atlantic specimens. The difference can not be 

 regarded as signifying local variation, for the representatives of the two classes 

 were selected from widely separated localities. 



Preserved specimens furnish less satisfactory data on the form of the limbus 

 than they do on the outlines and proportions of the shell. However, in very 

 large Pacific specimens (30-40 mm. long) preserved in formalin, in which this region 

 was in good condition, it is entire, the onlj- subdivisions being obviously acci- 

 dental tears, instead of irregularly lobed as it has so often been described for T'. 

 velella. In other respects, I could find no constant difference. 



The supposed difference in the form of the crest is not borne out by the 

 series studied, for in small specimens both from the Atlantic it is triangular. 

 Even in the Pacific species this character is transitional, the crest becoming 

 rounded in large specimens: but the triangular form seems to be retained 

 longer in them than it is in V. velella. 



