OF MICROPTERUS CINEREUS. 495 
to the kindness of Prof. Flower; 2nd, two crania of adult individuals procured by 
myself in the Strait of Magellan and on the west coast of Patagonia; 3rd, two skeletons 
of immature flying birds from the same regions; and 4th and lastly, two crania of young 
birds, still unfledged, which were shot while swimming in company with their parents 
in one of the harbours of the Messier Channel. 
At the outset I may remark that, as might be expected from the habits of the bird, 
the bones, with the exception of the skull and vertebra, appear to be entirely destitute 
of pneumatic apertures’. 
The cranium is of large size (upwards of five inches in extreme length, measured 
along the base in the largest adult specimens examined), and is very powerfully formed, 
with the various processes well developed, and the muscular impressions strongly 
marked. Its constituent bones appear to be unusually long in undergoing anchylosis, 
many of them readily separating in the process of maceration in fully fledged flying 
birds. The ridge circumscribing the occipital region is sharp and projecting in adult 
individuals, in which there is also a well-marked mesial vertical ridge above the 
foramen magnum. The foramen magnum is of large size; usually its upper border 
forms a deep angle; but in one specimen examined it is rounded in a manner similar 
to that which obtains in the Muscovy and some other species of Ducks. The lateral 
occipital vacuities vary much in respect of both size and shape in different individuals. 
That they are true “ fontanelles,” as termed by Meckel, and that they are not due, as has 
been asserted, to the influence of “the pressure of the brain within and the muscles 
from without,” is rendered plain by the circumstance that in young crania, in which the 
bones have only undergone a very slight amount of anchylosis, they are proportionally 
larger than in those of adult birds. Usually the fontanelle of one side (right or left, as 
the case may be) considerably exceeds that of the other in size, being sometimes more 
than twice as large, the result of ossification proceeding at an unequal rate. Occasionally, 
however, they are as nearly as possible of equal dimensions. In adult individuals the 
surface of the occiput on either side of these fontanelles is rough, indicating the attach- 
ment of certain of the muscles which move the head upon the neck. The usual 
foramina for the transmission of various small blood-vessels and the eighth and ninth 
pairs of nerves are present on either side of the occipital foramen and condyle, varying 
slightly in position in different specimens. The condyle presents the customary reni- 
form-shape, with a median vertical sulcus. The fossa subcondyloidea is deeply excavated, 
and of a rounded form. ‘The paroccipital processes are well developed, and more 
divergent than in some other Anatide, such as the Muscovy Duck. 
The elements of the occipital bone, 7. e. supra-, basi-, and exoccipitals, remain distinct 
and readily separable for a considerable period. In a skull of one of the youngest 
specimens, which measures two inches and a half in length, this is very well seen. The 
1 These apertures are more conspicuous in the Muscovy Duck (Catrina moschata) than in the skeletons of 
most of the other Ducks examined by me. 
4a2 
