224 ME. p. CHALMEES MITCHELL ON SO-CALLED 



there is an origin from the head of the fibula representing the 

 usual accessory tendinous head, and from this slips go to the 

 flexors of the toes. In D it is present, but very slender, above 

 the knee, and does not cross the knee-joint, being completely 

 absent below. In B and E it is quite absent above the knee, 

 but below is represented by the fibrous rudiment from the fibula 

 which I made knoAvii in Parrots and Herons. In A it is com- 

 pletely absent above and below the knee. 



Thus the eutaxic pigeons show a strong tendency to de- 

 generation in this muscle, which is generally present in a fairly 

 well-developed form among diastataxic forms of pigeons. It is 

 interesting to notice that in the eutaxic Opistliocomus, which in 

 many respects resembles pigeons, the ambiens is degenerate. 



ilf. peronetis profundus. — This muscle is very variable in birds, 

 and is present in all pigeons; but in the eutaxic forms it is 

 markedly degenerate. 



Visceral Anatomy. 

 I have found no indications of very great importance from 

 the point of view now under consideration. The oil-gland 

 is absent in only one of them (C), but it is small in B and 

 E. It is almost certain that the presence of lai'ge cseca 

 is a characteristic of the more primitive forms of the gut in 

 birds (7). In pigeons, generally, the c^ca are on the wane, but in 

 the great majority of forms they are definitely present as a pair 

 of nipple-like structures. Among tlie eutaxic forms they are 

 present in B, but extremely small and showing patches of dark 

 pigment as are to be found frequently on degenerate organs. 

 In C, according to Garrod, they are present ; in the only speci- 

 men I have seen they were absent, so that it is probable that, 

 individual variation occurs. In the other five eutaxic forms 

 they are totally absent. The gall-bladder is absent in all the 

 eutaxic forms, and as it occurs in a comparatively large number 

 of diastataxic forms, its absence may have some significance. 

 In all the eutaxic forms, the gut shows the arrangement of coils 

 and loops which I have described as typical for pigeons (7) in a 

 strongly -marked form. In A, o, B, and E the middle loop, so 

 characteristic of pigeons, is particularly long and shows the 

 peculiar spiral twisting in of the parallel distal and proxin al 

 limbs in a strongly-marked fashion. So far as the viscera are 

 concerned, it may be said with confidence that the eutaxic 

 pigeons exhibit the t) pical pigeon-variations in a high degree. 



