215 



indicate an advance beyond the Sauropsidan to a distinctly Mam- 

 malian status. 



2) The features A. 6, 8 and 9 mentioned above, are distinctly 

 Saurian, but the degree of elaboration of the hippocampal formation 

 is characteristically Mammalian. 



A. 6) The fact that the supra- and precommissural parts ot the hippo- 

 campal arc (especially in Ornithorhynchus) are larger and better de- 

 veloped than the descending part of the arc, is distinctively Pro- 

 totherian. 



A. 8) The round or oval shape of the commissura dorsalis (vel hippo- 

 campi) in sagittal section is characteristic of this order. 



A. 9) The attachment of the. roof of the forebrain to the anterior lip 

 of the thickened upper extremity of the lamina terminalis so as 

 to form a supracommissural diverticulum of the third ventricle (reces- 

 sus superior) is a significant feature which may be shared by some 

 marsupials [Didelphys ?, Perameles?, Notoryctes?, Phascolarctos?), 

 but is certainly not common to all the marsupials. 



3) The dwindling of the lateral parts of the cerebellum in the Mo- 

 notremata is a slight approximation to the Sauropsidan condition, 

 although the cerebellum in Monotremes is very much closer to the 

 Mammalian than it is to the Saurian type. 



4) The diminutive size of the geniculate bodies and the smallness 

 of the testes in the Monotremata probably indicate that the latter have 

 not yet completely attained to the fully-developed mammalian position, 

 but retain some suggestions of the Saurian status. 



5) The olfactory bulb and nerve in Ornithorhynchus afford a pe- 

 culiar instance of the persistence of the Saurian type. 



G. B. Howes 6 hat im Jahre 1887 darauf aufmerksam gemacht, 

 daß die Monotremen ein den Reptilien ähnliches Os epicora- 

 coideum besitzen: »It is now generally conceded that some of the 

 Anomodont reptiles, which in many respects so closely approxi- 

 mate towards the Mammalia in their skeletal anatomy, were possessed 

 of an expanded epico racoid of the Mono tr erne type.« 



H. G. Seeley 7 hat im Jahre 1896 ein fossiles Reptil mit 

 Ornithorhynchus verglichen und sagte : » Ornithorhynchus shows pre- 



6 G. B. Howes, The Morphology of the Mammalian Coracoid. Journ. of 

 Anat. and Physiol. Vol. XXI. 1887. — On the Mammalian Pelvis, with special re- 

 ference to the young of Ornithorhynchus anatinus. Anat. and Physiol. Vol. XXVII. 

 1893. 



7 H. G. Seeley, On the complete skeleton of an Anomodont Reptile [Aristo- 

 clesmus Rütimeyeri Wied.) from the Bunter Sandstone of Reihen, near Basel, giving 

 new evidence of the relation of the Anomodontia to the Monotremata. Proc. of the 

 Roy. Soc. for 1896. 



