AND MYOLOGY OK THE MARSUriALIA. 869 



lobes in either of these species, and I was unable to detect them 

 in the Didelphyidse, Dasyuridpe, and Peramelida3. He believes 

 that the primitive condition is present in the Dasynridse, and the 

 cervical lobes of the Diprotodonts are a specialization in asso- 

 ciation with the vegetable diet and the call for a larger amount 

 of thymic tissue. If, as Swale Vincent suggests, the cervical and 

 thoracic lobes perform different functions, it is difficult to ascribe 

 any use other than dietetic to the former. 



As age advances, the thymus — both cervical and thymic — 

 diminishes in size and degenei^ates. 



The Thyroid Qland. 



In JPerameles ohesula the left lobe is long and narrow, but the 

 right one is short and oval, and partly under cover of the pec- 

 toralis major. No isthmus is present ; and this arrangement is 

 the reverse of that which I have already described in Phasco- 

 larctos cinereus (15). 



In all otlier Marsupials, I found the gland to be composed of 

 two small, oval lobes lying on the sides of the larynx alone 

 (Petaurus sciureus), or the posterior part of the larynx and a 

 variable number of tracheal rings ; and the only animal possessing 

 an isthmus is Trichosurios vulpecula. 



Each lobe receives a single artery from the common carotid, 

 and the veins join the internal jugular veins. 



The lobes were concealed by the pretracheal muscles in all 

 animals examined except a male pouch specimen of Maa-opus 

 giganteus. 



The Spleen (text-fig. 76 G-J). 



The observations recorded here are supplementary to those of 

 Colin Mackenzie (5), Owen (11), and Retterer (21). 



In most genera there is little vai-iation in form in the various 

 species, but the spleen has different shapes in the species of 

 JJendrolagus. In D. ursinus it is long and Y-shaped, with 

 rounded extremities (text-fig. 76 1). In D. inustus (12) it is 

 T-shaped, and it is spoon-shaped in Z). bennetti (1). Carlsson (2) 

 states that it is pointed in D. doriamts. 



In Petaurus sciureus (text-fig. 76 J), P. breviceps, and P. hreviceps 

 pcqnianus the spleen is triangular, with sharp angles not prolonged 

 into processes, and the base of the triangle is applied to the 

 greater curvature of tlie stomach. 



The spleen of Phalanger orientcdis resembles those of the 

 Petaurists, the only difference being that its apical angle is 90°, 

 whereas those of the latter are larger. 



Colin Mackenzie described and figured the spleen of Macropus 

 and my specimens differ in no way from his description ; and a 

 pouch specimen of Onychogale lunata has a Y-shaped spleen 

 similar to that of the Kangaroos. The form does not alter from 

 birth to adult life. 



