12 PROF. W. N. PARKER ON THE [Jan. 16, 



be seen that the young Echidna resembles Ornithorhynchus in 

 possessing : — 



1. A thick horny layer covering the snout, as well as horny 

 teeth and a horny caruncle for breaking open the egg-shell. 



2. Glands resembling ordinary sweat-glands opening on the 

 snout as well as on the anterior part of the palate. 



3. A highly developed Jacobson's organ, resembling that of 

 Lizards and Snakes, enclosed in an independent tubular cartilage, 

 and possessing a large " turbinal " supported by a cartilaginous 

 shelf continuous with the investing tube. 



4. A complicated " maxillo- turbinal." 



5. A communication between the two nasal cavities, as in 

 certain birds. 



6. Numerous glands in connection with the nasal chamber and 

 Jacobson's organ, including a specially large " septal gland " and a 

 " Steno's gland." Most of these characteristics are peculiar to 

 Monotremes amongst Mammals. 



On the other hand, Echidna differs from Ornithorhynchus in : — 



1. The absence of any rudiments of true teeth in the young of 

 12 cm. in length and onwards, and the early extreme specialization 

 of the entire mouth-cavity. 



2. The possession of a mammary pouch in the young female. 



3. The less solid character of the nasal capsule, the much higher 

 development of the ethmoid turbinals, and the absence of transverse 

 connective-tissue septa in the anterior part of the nasal cavity. 



4. The slightly smaller relative size of Jacobson's organ and of 

 its turbinal, the organ also not extending anteriorly to the naso- 

 palatine canal. 



Since his communication already referred to, Dr. Symington ' 

 has shown that the Jacobson's organ of Marsupials conforms to the 

 general type met with in the Eutheria, and thus differs markedly 

 from that of the Prototheria. The former may very probably 

 have arisen from the latter, but it has undergone various degrees 

 of degeneration : a very slight step in this direction has possibly 

 occurred in Echidna. Symington is probably correct in his 

 opinion that Jacobson's organ reaches its highest deA^elopment in 

 the Monotremes — higher even than in Lizards and Snakes, in which 

 it presents many points in common with that of the Prototheria. 



It certainly seems probable that " Ornithorhynchus is far the 

 most primitive type " 2 of the two Monotremes : the young Echidna 

 resembles Ornithorhynchus much more than does the adult, and is 

 very highly specialized as regards many characters besides those 



1 " On the Organ of Jacobson in the Kangaroo and Rock Wallaby (Maeropus 

 giganteus and Petrogale penieillata)," Journ. of Anat. and Physiol, vol. xxvi., 

 n. s. vol. vi. 1892. 



Quite recently Rose has described the Jacobson's organ in embryos of the 

 Wombat and Opossum, and has shown that in the former its duct is situated 

 on the floor of the organ as in Ornithorhynchus, and not at its anterior end ; 

 and also that a large mucous gland is connected with its posterior end (Anat. 

 Anz. viii. Jahrgang, 15 Sept. 1893, p. 766). 



2 W. K. Parker, he. cit. p. 36. 



