OCULAR AND GENITAL PLATES. 135 



110. Dumpling Islands, Maine. Diam. 58 mm. R. T. J. Coll., 853. X 3. Oculars V, II insi-rl, an incomplete I, V, 

 II, due to the exclusion of ocular I by the fusion of genitals 5, 1. Madreporic pores are in genital 2 and ocular III (p. 172). 



147. Dumpling Islands, Maine. Diam. 54 mm. R. T. J. Coll., 854. X 3. Oculars I, IV, II insert, an incomplete 

 I, V, IV, II, due to the exclusion of ocular V by the fusion of genitals 4, 5; very rare variant. Ocular III is opposite geni- 

 tal 2 only, not at the junction of genitals 2 and 3, as usual (p. 167). 



148. Caldervvood Island, Maine. Diam. 50 mm. R. T. J. Coll., 855. X 3. Oculars V, IV, II insert, an incomplete 

 I, V, IV, II, as in text-fig. 147, but due to the exclusion of ocular I by the fusion of genitals 5, 1, a very rare variant (p. 167). 



149. Dumpling Islands, Maine. Diam. 9.5 mm. R. T. J. Coll., 856. X 12. Oculars I, V, IV, III insert, a very rare 

 variant. Genitals 3 and 4 split by secondary sutures (compare text-fig. 69, p. 98). 



and family. It is a right-handed development of three plates insert, which is usually expressed 

 as I, V, IV insert. One case of I, IV, II (text-fig. 147) and two of V, IV, II insert (text-fig. 

 148) were found, both very rare variants; in these an ocular of the bivium is mechanically shut 

 out by the fusion of the adjacent genitals. In two cases oculars I, V, IV, III are insert (text- 

 fig. 149), and these were the only cases found in which ocular III entered the periproct in this 

 species. 



From the above it is seen that in the 33,000 specimens there are 402 aberrants, which is 

 1.2% as stated. Of these, 215, or more than half, have I, V, II insert, and 92 are cases in which 

 the fusion of two genitals excluded an ocular causing the irregularity. This leaves only 92 or 

 0.28% as somewhat scattered aberrant variants. 



Returning to the Dumpling Islands series, we find that those exceeding 30 mm. in diameter 

 are considered as the developed series because the percentages are virtually the same for all 

 specimens over this size, and differ from those for specimens under this size as such are still de- 

 veloping. This knowledge can only be attained by studying large series, where the possible 

 irregularities of a small series are eliminated. 



The series of 30 to 40 mm. in diameter (1,750 specimens) has 2.4% with ocular I insert; 

 95% have I, V; and 0.9% have I, V, IV insert. The aberrants are unusually frequent in this 

 lot, 36 specimens or 2%. Of these variants, ten specimens have oculars I, IV insert; three 

 have V, II; five V, IV; and eighteen I, V, II insert. 



The series of 40 to 50 mm. in diameter are about the size of the larger specimens met with 

 in most localities. Of this size (2,900 specimens) 2.8 % have ocular I only insert, a relatively 

 high number, but on the principle of chances some fluctuation must be expected. One speci- 

 men has V only insert, being one of two such found in the whole lot. In 95 % oculars I, V, and 

 in 1.2% oculars I, V, IV are insert. The aberrants are 2%. Of these, three specimens have 

 oculars I, II insert; nine have I, IV, three have V, II, two have V, IV, and 27 have oculars I, 

 V, II insert. 



The series of 50 to 60 mm. in diameter may be considered large specimens. Of this size 

 (1,272 specimens) 2.4 % have ocular I only insert; 94 % have I, V, and 1.3 % have I, V, IV insert. 

 The arrested and progressive variants are practically the same as in the 30 to 40 mm. series, 

 and the progressive variants as in the 20 to 30 mm. series. The aberrant variants are 2%. 



