428 DB. B. c. A. WINKLE OS DOUBLE [May 21, 



formation of the jaws, as in the following cases : — (a) In this case 

 the smaller head was unprovided with eyes and the lower jaw was 

 very short and tapered rapidly to a sharp point. (6) In this case 

 the upper jaw was much shorter than the lower, which projected 

 a long way in front of it : there was, in fact, an almost 

 complete failure to develop of that portion of the head which is 

 placed in front of the eyes, (c) In this case the condition was 

 carried still further, for so extensive was the arrest of the develop- 

 ment of the anterior part of the skull that the two eyes were 

 placed quite close to one another, with scarcely any separation 

 between them. These two last forms approximate to the condition 

 known as cyclopia. 



3. Frequency of occurrence. Jacobi, as has already been men- 

 tioned, stated that double monstrosities occurred more frequently 

 in some years than in others ; and I remember the late Mr. Burgess, 

 the founder of the lish-hatching establishment at Malvern, making 

 exactly the same remark to me, though he was unable to assign 

 any cause for the variation. They have been met with amongst 

 Sharks, Mackerel (15), Salmon, Trout, Perch, and Pike. I do not 

 know M'liether any statistics have been drawn up at any of the fish- 

 hatching establishments as to the percentage of double monsters to 

 normal forms, but the following figures have been obtained from 

 data in some of the papers to which I have referred : — 



Observer. Fish. No. Examined. No. of Double monsters. 



Eauber. Trout. 1000 2 



Pike. 325 1 



Coste (16). Various. 400,000 Over 100 



Lereboullet. Pike. 203,962 222 



Prom the last figures, which are the most complete, it would 

 appear that the percentage of occurrence in the pike is rather less 

 than 'l, or about one per thousand. 



With regard to the frequency of occurrence of the various classes 

 into which I have divided these forms above, the distribution in 

 the cases which I have examined myself is as follows : — 



1. Three eyes, all one size 3 



2. „ median being larger than others 1 



3. Pour eyes 1 



4. Two heads 



5. Pission extending to the pectoral region 11 



6. „ extending to posterior border of yolk-sac .... 7 



7. „ extending beyond posterior border of yolk-sac 10 



8. Tails overlapping 9 



9. Union only by tails 



10. Anakatadidymus 2 



11. Parasitic 2 



