734 AN ECHIDNA WITH EIGHT CERVICAL VERTEBRAE. 



zygapophyses. The dorsal vertebra in Echidna usually have the 

 arches perforated for the passage of the spinal nerves, but this 

 character is generally absent in the first dorsal, and in one speci- 

 men in my possession is absent in the first five dorsals. In this 

 abnormal eighth vertebra there is no indication of a perforation 

 in the arch or even of a distinct notch for the nerve. 



In Echidna, as in Ornithorhynchus, the ribs have almost 

 completely lost the double-headed character so well seen in their 

 Theriodont ancestors and retained in the large majority of 

 mammals. In the first rib there is usually an indication of the 

 double-headed condition. In this abnormal eighth vertebra there 

 is no difficulty in recognising the double articulation of the rib 

 with the centrum and with the transverse process. Usually the 

 first rib articulates not only with the first dorsal, but also with 

 the seventh cervical vertebra; the short ribs of this specimen 

 articulate with the eighth vertebra alone. 



The rib of the right side tapers to a point, but the left rib is 

 slightly dilated at the end and forms an articulation with the 

 side of the first fully developed rib. 



The rib of the ninth vertebra, which is the first to meet the 

 sternum, does not articulate with the anterior broad part of the 

 manubrium sterni as normally, but with its hinder part, almost 

 as the second rib does usually. 



The specimen is interesting as showing that even the cervicals 

 may be subject to variation in this genus where variability seems 

 to be so very common. 



