EAR-BONES OF OPFOSSUM. 105 



tympanic bone is not yet formed, its future position being indi- 

 cated, however, by a definite mass of cells, situated between the 

 eustachian tube and the gelatinous anlage of the meatus audi- 

 torius externus, and just below the anlage of the membrana 

 tympani (Plate II). Secondly, Professor Minot quotes Parker 

 to the effect that an investigation of the Marsupialia had con- 

 vinced him of the error of his previous opinions, since the incus 

 was here without any doubt continuous with the mandibular 

 arch. Parker's investigations upon this point have, however, 

 never been published ; as far as Parker ventures in the brief 

 statement above referred to, my observations perfectly coincide 

 with his. 



The material at my disposal consisted of three Opossums of 

 the species, DidelpJiys vmrina of Trinidad.^ One of these, 

 the largest, was of no service in the research, as the sections 

 revealed that the ear-bones had already assumed their adult 

 relations. The other two, coming from one litter, were of the 

 same size, and represented identically the same stage of devel- 

 opment. They were taken from the pouch, but measured in 

 length no more than six to seven millimeters. The length of 

 the head from the tip of the snout to the external ear was two 

 to three millimeters. Sections of the thickness of seven p. were 

 cut with the Minot microtome, and the complete series mounted. 

 In order to demonstrate the relations of the cartilages, a model 

 of the ear bones of the left side was made, in which all their 

 dimensions were multiplied by 175. The method here employed 

 was a slight modification of that recommended by His. 



An understanding of the general topographical relations may 

 be gained from an inspection of the figure of the model, the fol- 

 lowing description being added merely as commentary. From the 

 distal extremity Meckel's cartilage passes up on each side at an 

 angle of about 30° to the base of the skull, lying inwards of the 

 anlage of the dentary, to which it contributes in the usual man- 

 ner. The shape of the cartilage towards the symphysis is 

 rounded but as the sections run back, it is seen to increase very 



1 1 am happy to make acknowledgement of the courtesy of Dr. Chapman and 

 Professor Allen to whom I owe this material. 



