104 BULLETIN 60^ UNITED STATES NATIONAL MUSEUM. 



three-quarters of a century ago. The hiter accounts in the mono- 

 graphs of Darwin and Gruvel are compiled from Quoy and from 

 Lesson, who described a similar form, perhaps specifically identical. 

 According to Quoy. the tunic is gelatinous and diaphanous; the cirri 

 are short, quite broad, and straight, white (though colored blue on 

 the plate in the Astrolabe voyage), and composed of about 10 seg- 

 ments. An isolated cirrus figured shows rami w^ith 12 segments. 

 The total length is nearly 2 inches. Nothing is said of caudal append- 

 ages,^' 



This account, so far as it goes, applies so well to the Pacific species 

 described below that I can not doubt that the two forms are very 

 closely related and possibly identical specifically. It is, moreover, 

 evident that Gymnolepas i^elludda Aurivillius, based upon a specimen 

 about 1 inch long, is closely related to A. U7iivalvis^ though probably 

 distinct specifically. The genus will therefore include three species: 



Alepas univalvis (Quoy and Gaimard). Eastern Atlantic. 



Alepas pellucida (Aurivillius). North Atlantic. 



Alepas pacifica Pilsbr3^ Eastern Pacific. 



Alepas^ as here restricted, differs from the genus Heteralepas by its 

 thin-walled integument, the absence of segmented caudal appendages, 

 and by the short, few-jointed, and weakly chitinized cirri. These are 

 adaptive characters, correlated with pelagic conditions. 



The arrangement of spines on the cirri is much alike in Alepas and 

 Paralepas. In A. pacifica there is, on the more anterior cirri, a 

 complete circle of spines around the distal end of each segment. On 

 the last two or three cirri the circle is more or less interrupted, both 

 on the outer and inner faces of the rami. This is most pronounced in 

 the sixth cirrus, the fifth segment of which is illustrated in fig. 34 G^ 

 page 100. Except that the first cirrus is more profusel}^ spinose along 

 the anterior side, it does not difi'er in armature from the adjacent cirri. 

 Aurivillius has figured a cirrus of his GyinnolejMS pellucida with the 

 same arrangement of spines described above for A. pacifica. 



« Darwin in/ej's the presence of "a pair of long articulated caudal appendages" 

 from Lesson's statement that his Triton {Alejms) fascicuhdvs has seven pairs of cirri. 

 In no allied form are the caudal appendages so large as to be mistaken for cirri, and 

 it is far more probable that Lesson miscounted the cirri — a mistake very easy to 

 make where they lie limp and tangled, as I have noticed in A. pacifica. The infer- 

 ence that A. univalvis {A. parasita) has caudal appendages depends, moreover, upon 

 the identity of the form badly described by Lesson with that of Quoy and Gaimard — 

 a proposition which can not be considered as demonstrated. In Gruvel' s mono- 

 graph Darwin's inference as to presence of caudal appendages is adopted as a matter 

 of fact. 



