76 RICE. [VOL. II. 



the gill proper to the appendage. This line again marks a partial 

 or total obliteration of the folding of the gill - - a point of impor- 

 tance in the consideration of the meaning and cause of the 

 fusion of the filaments. 



The term "fusion of filaments" has been employed repeat- 

 edly ; but the question arises whether the phenomenon under 

 discussion is really a fusion of a large number of once distinct 

 filaments or a branching of a relatively small number of original 

 filaments ; whether, in other words, the primary filaments are 

 represented by the maximum or minimum number. The answer 

 is not far to seek. At the free margin of the gill the continuity 

 of the filaments may be readily traced from one lamella to the 

 other, and the number of filaments in the two lamellae within 

 any given fold must necessarily be equal. A little higher, in the 

 zone of fusion, these numbers may become very unequal. For 

 example, in a section of the gill of Batissa seven filaments of 

 one lamella were observed to correspond to twelve in the other. 

 Neither of these facts, however, offers conclusive evidence ; both 

 may be explained as well on the supposition of branching as on 

 that of fusion. But as the serial sections are followed a little 

 higher in the gill, the inequality in the number of filaments, which 

 has increased from zero at the margin to a maximum in the zone 

 of fusion, begins to fall off again more or less rapidly, and finally 

 reduces itself once more to zero. This equality in the number 

 of filaments in the upper parts of the gill can be explained only 

 on the supposition that the phenomenon before us is a fusion, 

 not a branching, of the gill filaments. It is altogether too im- 

 probable that the independent branching of the filaments in the 

 two lamellae would lead to the same number in the two cases. 



Granted that the phenomenon is a fusion, what is its mean- 

 ing ? Is it of systematic importance ? My observations are 

 not complete enough to permit a categoric answer ; but I am 

 strongly inclined to the belief that the fusion carries no more 

 of weight from the systematic standpoint than does the folding 

 of the gill. 



Among smooth gills no fusion was observed, although a con- 

 siderable number of forms were studied. Especial attention 

 was devoted to Tellina and Scrobicularia, in which, for reasons 



