CONTRIBUTIONS TO THE STUDY OF VARIATION. 59 



15mm. long is "75mm. The apex is swollen into a somewhat conical 

 bulbous form, and beneath this is a narrow constriction, followed by 

 another slight swelling, which passes gently into a short cylindrical 

 stalk (Fig. 1). 



Of abnormal antennas, there was a perfect series grading from 

 the normal form as described, to one that could scarcely be distin- 

 guished either in size or shape from that of a perfectly formed 

 normal operculum. These gradations are outlined in Figs. 2 to 10. 

 Fig. 2 represents a form common to several specimens. In it the 

 equator of the terminal swelling is scored by shallow vertical grooves. 

 The next stage (Fig. 3) is where the equator besides being grooved, 

 is slightly raised into a serrulate and grooved rim. In Fig. 4, is a 

 form where this is accentuated, while the constriction beneath the 

 terminal swelling is much more emphatic. Another specimen had 

 the antenna (Fig. 5) one-third the length of the operculum, the 

 terminal knob being modified into a miniature of a true opercular 

 upper cup, and hiding entirely — for the first time in the series — 

 the conical apex of the organ. 



In all these instances, the lower swelling was nearly normal. 



Following on Fig. 5, we have a form such as Fig. 6, where, with 

 as well developed an upper cup as in the preceding, the lower has also 

 started to develop, taking the form of an obscurely serrated collar 

 at the base of the upper cup. 



Finally in Figs. 7, 8, 9 and 10 we have a number of cases where 

 both cups are well developed, and closely approach the form of the 

 normal operculum. Keeping pace with this series of gradations in 

 development, was the growth in length of the stalk of the pseudo- 

 operculum. That shown in Fig. 2 was rather stouter, but not 

 much longer, than the normal form of antenna, 4 was rather longer, 

 5 was one-third the length of the operculum, while the remainder 

 varied from one-half to all but equal length with the same organ. 



The forms shown in Figs. 2, 7, 8, 9 and 10 — those at the begin- 

 ning and the end of the series — were the most common. 



Summing up the foregoing facts we find that : — 



a. The operculum is developed on either the right or the left 



antenna inconstantly. 



b. The non-opercular antenna is normally cirrus-like with two slight 



swellings towards the further end. 



c. The same organ in 25 °/ of instances is abnormal, showing all 



manner of degree of approximation to the form and size of 

 the functional operculum. 

 Before considering further the meaning attaching to the varia- 

 tions as above set forth, let me enumerate the principal points 



