8 ABNORMAL CONDITIONS OF THE AXIS. 



exposed. The upper mandible and maxillary bones are 

 entirely deficient, but there is a slight frontal protu- 

 berance. Presented by D. Hanbury, Esq. 



Subclass II. Homologous union. 

 a. Pleuropagus, Forster. 



The following preparations exhibit posterior combined with 

 anterior dichotomy ; but there appear to be two axes throughout, 

 although the vertebrae are themselves fused in some part of the 

 spinal column. The condition will be at once understood by 

 comparing Nos. 118 to 121. It is impossible, as has already 

 been stated, to say whether the condition arises from fission or 

 fusion. 



42. Twin foetal Pigeons united by the dorsal regions of the 



spinal column, the median visceral laminae being absent. 

 The right lamina of one bird and the left of the other 

 form the abdominal and thoracic walls. The heads and 

 necks are distinct. The neural arches in the upper part 

 of both vertebral columns are defective. 



Purchased, 1858. 



43. Twin foetal Guinea-fowls similarly united with fusion 



of the vertebral columns in their upper part. There is 

 also fusion of the head and necks. The median eyes are 

 fused into a single but abnormally large organ. The 

 lower mandibles likewise form a single organ. The neural 

 arch of the skull is open, and the brain and spinal cord 

 are deficient. Presented by E, Belfour, Esq. } 1818. 



44. A similar union of twin foetal Ducks. The brain is com- 



pletely double and without any covering. 



b. Hypogastrodidymus, Gurlt. 



The two succeeding specimens almost exhibit the characters of 

 heterologous union ; they, however, probably commenced their de- 

 velopment by homologous union, and have therefore been placed 

 in this Subclass. (See also Nos. 103 and 104, which present a 

 similar condition.) 



45. A Chick with the rudiments of a second Chick occupying 



the pubic symphysis. The rudimentary Chick consists 



