1895.] MALFOEMATIOXS AMONGST PISHES. 425 



■which have been described by Enoch, von Baer, Klaussner, St.- 

 Hilaire (10), and Lereboullet (11). 



5. Duplicity extending to the region of the pectoral fins (fig. 3). 

 I have se\'eral instances of this condition, which lias also been 

 described by von Baer and Eauber (12). In these cases, as can be 

 seen when the yolk-sac is still present in the specimen, the division 

 extends as far back as the anterior border of that appendage. 1 

 may here mention, as some stress has been laid upon that point by 

 de Quatrefages (13), that, although I have carefully looked for it, 

 I have never found any sign of a notch or fissure at the anterior 

 border of the yolk-sac in these or any other of my specimens, such 

 as he saw in some of his and believed to be an indication of the 

 union of two originally distinct sacs. 



6. Duplicity extends to the posterior border of the yolk-sac, the 

 caudal extremity of the fishes being quite single (fig. 4). 



7. Duplicity extends a short distance behind the posterior border 

 of the yolk-sac, so that there is a triangular gap between the sac 

 and the adjacent sides of the two bodies. The caudal extremity is, 

 however, quite single. I have several specimens of each of the 

 conditions described in this and the preceding class. 



8. Duplicity extends to the posterior border of the yolk-sac. 

 Behind this there are two caudal extremities overlapping one the 

 other and firmly united by their contiguous aspects. Each is pro- 

 vided with a distinct and independent caudal fin, a point easily 

 overlooked in a cursory examination, since one overlies the other 

 (fig. 5). In these cases there are two vertebral columns; and from 

 the relation of the caudal ends to one another, it would appear 

 that at some period of development they had been separate, and 

 had subsequently fused in part with one another. I have several 

 specimens belonging to this class, and the condition has also been 

 described and figured by Eauber. 



9. Union by the caudal extremities alone. I have not seen this 

 condition, which has been noted by Klaussner, Lereboidlet, and 

 Eauber. 



10. Union is by the ventral aspects at the site of attachment of 

 the yolk-sac (anakatadidymus). I have specimens of this class, 

 which has also been described by de Quatrefages and Valentin (14). 



In the above-mentioned cases the two portions, greater or 

 smaller, of which the double monster consisted are of approxi- 

 mately the same size. As a matter of fact, the occurrence of two 

 united fishes, each being of exactly the same size, must be an 

 occurrence of some rarity. I find it in none of my specimens, and 

 in by far the greater number of recorded cases the inferiority in 

 size of one member is especially mentioned. There is, however, no 

 such marked difference in size as is met -tt-ith in another group, 

 which should now be considered, that, namely, of 



11. Parasites, as they may be called, adopting a term familiar to 

 teratologists. In these cases one member is reduced to the con- 

 dition of a mere appendage to the larger and more perfect half. 

 A few instances of this condition may be briefly described from 



