DOUBLE MONSTERS IN FISHES 53 



Type 2. Union in pectoral region: 



a) The pectoral fins absent on adjacent sides. 



h) The pectoral fins present, but united on adjacent sides. 

 Type 3. Union behind the pectoral region: 



a) The twin bodies united at a considerable distance in 

 front of the vent. 



b) The twin bodies united close to the vent. 



Type 4. Union by yolk sac only. 



The first three types should be classed as conjoined 

 twins or double monsters and the fourth type as sepa- 

 rate or duplicate twins. The series is evidently a very 

 complete one in which every degree of duplicity of the 

 primary axis, from a slight twinning in the most anterior 

 region to complete separation, is found. There are no 

 cases of double tails unless the twins are entirely separate. 



By far the commonest type of double monster is 

 that in which the process of dichotomy extends well 

 into the abdominal region, but not past the pelvic region. 

 Such an individual is shown in Figure 29 (p. 54). As 

 a rule individuals showing this degree of duplicity show 

 approximately equal development in the two anterior 

 components. When the duplicity extends farther toward 

 the posterior end, as in Figure 30, there is a greater 

 tendency for one component to be abnormal. Cases in 

 which the duplicity is confined to the most anterior struc- 

 tures are relatively infrequent. Such a type is shown 

 in Figure 31, in which the duplicity is confined to the 

 forebrain, and there is but one median eye, belonging 

 partly to one and partly to the other incompletely 

 double head. There are probably cases of twinning in- 

 volving even less than this, but no good figures of such 

 were to be found. 



