STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 837 



(2) The small size of the pancreas, and the presence of 

 numerous so-called pancreatic caeca. 



(3) The somewhat coiled small intestine. 



(4) Certain characters of the brain, e.g.. the large size of 

 the cerebellum ; the presence of the so-called lobi inferiores 

 on the infundibulum ; and of tori semicirculares in the mid- 

 brain. 



In spite of the undoubtedly important list of features to which 

 we have just called attention, a list containing not less important 

 characters, both embryological and adult, separating Lepidosteus 

 from the Teleostei, can be drawn up : 



(1) The character of the truncus arteriosus. 



(2) The fact of the genital ducts joining the ureters. 



(3) The presence of vasa efferentia in the male carrying the 

 semen from the testes to the kidney, and through the tubules of 

 the latter into the kidney duct. 



(4) The presence of a well-developed opercular gill. 



(5) The presence of a spiral valve; though this character 

 may possibly break down with the extension of our knowledge. 



(6) The typical Ganoid characters of the thalamencephalon 

 and the cerebral hemispheres (vide pp. 769 and 770). 



(7) The chiasma of the optic nerves. 



(8) The absence of a pecten, and presence of a vascular mem- 

 brane between the vitreous humour and the retina. 



(9) The opisthoccelous form of the vertebrae. 



(10) The articulation of the ventral parts of the haemal arches 

 of the tail with processes of the vertebral column. 



(11) The absence of a division of the muscles into dorso- 

 lateral and ventro-lateral divisions. 



(12) The complete segmentation of the ovum. 



The list just given appears to us sufficient to demonstrate 

 that Lepidosteus cannot be classed with the Teleostei ; and we 

 hold that Muller's view is correct, according to which Lepidosteus 

 is a true Ganoid. 



The existence of the Ganoids as a distinct group has, how- 

 ever, recently been challenged by so distinguished an Ichthyolo- 

 gist as Giinther, and it may therefore be well to consider how 

 far the group as defined by Mliller is a natural one for living 



