OF ECHIDNA AND ORNITHORHYNCHUS. 3 
Their inner margins articulate with, but posteriorly extend beyond, the copula. 
Broad at first, they narrow as they proceed backwards, and, embracing the cricoid, 
terminate underneath the more internal of the posterior processes of the anterior 
cornua. 
In Ornithorhynchus, the copula of the thyroid is smaller and narrower. Its 
anterior cornua come into contact, to a very slight extent, with the body of the hyoid, 
and to a greater extent proportionately than in Echidna with their own copula. 
Their anterior margins, at first in contact with the posterior hyoid cornua for a 
short distance, are next separated from them by an ovoid space occupied by 
membrane, and again, as we pass outwards, come into close contact and union with 
them. In this region the thyroid cornua, narrow anteriorly, expand into a broad 
plate in whose posterior margin a semicircular notch leaves two square posteriorly- 
directed processes. A strong ridge passes backwards from the line of junction with 
the hyoid cornua to a point internal to the inner process. The posterior thyroid 
cornua, attached to the copula by a narrower origin, corresponds in its other 
relations to the same part in Hchidna. 
The Cricoid Cartilage. 
In Echidna, the cricoid forms an imperfect ring of considerable breadth, and of 
the same diameter as the trachea. The anterior rings of the trachea are imperfectly 
defined, and are incomplete dorsally ; a broad notch of equal breadth is cut out of 
the dorsal aspect of the cricoid. The surface of the cricoid is marked by parallel 
transverse grooves, which do not meet in the middle lines. Anteriorly, close to the 
arytenoids, this broad notch is continued into a narrow slit. Two small procricoid 
cartilages are present in Echidna. The posterior one lies over the narrow anterior 
part of the notch in the cricoid, and broadens in front where it lies partly over the 
arytenoids. The anterior procricoid is a |-shaped piece, lying transversely across 
the anterior end of the posterior. 
In Ornithorhynchus, the upper rings of the trachea are also imperfect, but 
their ends are separated by a narrower interval. The notch in the cricoid is broader 
than the interspace between the ends of the tracheal rings, and does not extend to 
the anterior border of the cricoid. We did not detect procriccid cartilages similar to 
those in Echidna, but two small paired cartilages, evidently separated off from the 
arytenoids, lie dorsally between the latter and the anterior border of the cricoid. 
Nothing remarkable, save their large size, is to be found in the arytenoid 
cartilages of either form. 
Our specimen of Ornithorhynchus (an adult female) differs according to the 
above description from that figured by Dusors. In his specimen the two pro- 
cricoids were distinct, the body of the hyoid was larger and broader, and the 
