TRANSACTIONS OF SECTION D. 697 



The gaps between the lower classes are very great. The least differences 

 Ijetween the Selachians and Teleostomes are manifest in the Xenacanthini and 

 Dipnoi of the Palaeozoic ; the least between the Teleostomes and Amphibians in 

 The Crossopterygians and Stegoceplials. The differences between the Amphibians 

 and Reptiles are minimised in the Palaeozoic. From a generalised stock of the later 

 Palaeozoic or earlier Mesozoic the Mammals were doubtless derived. 



3. On the Derivation of the Pectoral Member in Terrestrial 

 Vertebrates. By Professor Theodore Gill. 



All attempts at the homologisation of the chiropterygium or anterior limb of 

 the pentadactyle or terrestrial vertebrate with the ichthyopterygium or pectoral 

 ■fin have been more or less unsatisfactory. The most important hint seems to be 

 furnished by Pohjpterus. Attention was called to the homologies by the author in 

 his Arrangement of the families of Fishes (1872) and the Standard (or Riverside) 

 Natural History. Similar conclusions have since been reached by others. The 

 <;hief objection to the derivation of the chiropterygium from the pectoral member 

 of such "a form as Polypterns is that at present no extinct representatives are 

 known. Probably future research will reveal such, as the genus belongs to a very 

 archaic type, and has numerous not very distantly related precursors in the past. 

 The homologies in question are justified by the facts of individual development of 

 the fore-limb in the Reptiles and Mammals. 



4. The Morphological Significance of the Comparative Study of 

 Cardiac Nerves. By Dr. W. H. Gaskell, F.R.S. 



•5. Observations tqwn the Morj)hology of the Cerebral Commissiires in 

 the Vertebrata. By Dr. G. Elliot Smith, M.A. 



6. Some points in the Symmetry of Actinians. 

 By Professor J. P. McMuekicii. 



7. The Natural History of Instinct. 

 By Professor C. Lloyd Morgan, M.A. 



8. On the Ilcematozoon Infections in Birds. By W. G. MacCallum, B.A. 



Several varieties of birds — crows, owls, sparrows, blackbirds, &c. — have been 

 found infected with organisms resembling the malarial organisms of man. These, 

 like the malarial parasites, develop within the red corpuscles, transforming the 

 haemoglobin into pigment granules. They reproduce by segmentation, although 

 this process does not occur simultaneously for great groups of individuals, so as to 

 make the length of the cycle of development easy of determination, as is the case 

 in the malarial parasites. The young hyaline forms are not actively motile. 



Two types of organism are recognised. In one (the Proteosoma of Labbe) 

 the irregularly shaped body is situated at one end of the nucleated red corpuscle, 

 displacing the nucleus toward the opposite end. This form segments in the peri- 

 pheral circulation. In the other type, which corresponds to the Ilalteridium of 

 Labbe, the body of the organism is elongated and curved about the nucleus of the 

 corpuscle. Segmenting forms are found in the bone marrow, but not in the circu- 

 lating blood. There is in this type a certain variation in form in different hosts. 



