392 bulletin: museum of comparative zoology. 



colorless, or pink. But observation of the living specimens showed that 

 the proportional length of stomach to lappets may change through wide 

 limits from moment to moment as the animal contracts and expands. 

 In a specimen 69 mm. long, at rest the proportion was almost exactly 

 what A. Agassiz shows ('65, fig. 19). The slight difference in the 

 length of the ribs may be nothing but a growth character at any 

 rate it is too slight to be used as the basis for specific separation. 

 There remains, then, only the question of color; and until the Pacific 

 form is known from more than one locality, the importance of this 

 must remain in doubt. It may be nothing more than a temporary 

 physiological phenomenon controlled by food; and this would be the 

 natural explanation did the canals alone show it, but the yellow color 

 of the tentacular apparatus may be more significant. B. vitrea is 

 described by A. Agassiz as wholly colorless. Two courses are open; 

 to refer the specimens provisionally to vitrea, or to institute for them 

 a new variety of the latter; the former will most satisfactorily express 

 our present scanty knowledge. 



One can not compare the Acapulco specimens with my ( : 04) figure 

 of ovalis without being struck by the likeness between the two; in- 

 deed the only apparent difference is that the latter, like vitrea, is 

 colorless. My only reason for separating it from the latter was that 

 the lobes were proportionately longer, the stomach shorter. Un- 

 fortunately the single specimen of ovalis was two fragmentary for 

 accurate diagnosis. But its obvious resemblance to vitrea suggests 

 at least the possibility that some variety of the latter will be found to 

 be at home in the tropical and subtropical waters of all three great 

 oceans. 



A comparison between vitrea and hydatina shows that the two are 

 closely allied in structural characters, i. e. they agree in the simplicity 

 of the lappet-canals and short auricles. But to judge from Chun's 

 figure ('80, taf. 4, fig. 5) the Mediterranean form is shorter, broader, 

 and less flattened than vitrea, and the ribs are rather shorter. We 

 have no account of the final stage in growth; the largest specimens 

 observed by Chun being only 4 mm. long; vitrea attains a length of 

 upwards of 70 mm. It may be that the differences in external form 

 are growth characters, and that such is the case is suggested by the 

 fact that I have compared small specimens from Acapulco with 

 corresponding stages of hydatina from Naples, without finding a 

 single character to separate the two. But it is hardly worth while to 

 speculate further along this line until living or well preserved adult 

 specimens from the Mediterranean are examined. I believe, however, 



