MONOTREMATA. 



3G9 



nections, and the proportions in which they 

 enter into the formation of the skull, have been 

 in great measure determined and described in 

 that genus.* 



I have had the opportunity of investigating 

 the composition of the cranium, a point so im- 

 portant in regard to the natural affinities of the 

 Monotremata, in the young Ornithorhynchi 

 transmitted to the Zoological Society of Lon- 

 don by Dr. Weatherhead ; and the comparison 

 of this part of their anatomy lias enabled me 



Fig. 169. 



better to appreciate and understand peculiarities 

 of the same part in the Echidna, the skull of 

 which is here also described from original spe- 

 cimens. 



In the cranium of a young but full-grown 

 specimen of Echidna setusa, (g,j!g. 169, 170, 

 171,) the four elements of the occipital bone are 

 unanchylosed and are joined together by smooth 

 linear harmonise. The basi-occipital (Jig. 170, a) 

 presents a six-sided rhomboidal figure, with the 

 posterior margin notched to complete the lower 



Skull of Echidna setosa. (Original.) 

 Fig. 170. 



boundary of the large vertical occipital foramen, 

 and thickened and smoothly rounded to form the 

 inferior extremities of the two occipital con- 

 dyles. These condyles are principally deve- 

 loped from the ex-occipital elements (Jigs. 169, 

 171, 6, 6), which are expanded superiorly 

 and terminate in an angle wedged in between 

 the supra-occipital and petrous bones ; they 

 extend, but do not meet, above the occipital 

 foramen, being separated by a notch closed by 

 membrane in the recent state. The supra- 

 occipital element (Jigs. 169 & 171, c) is a 

 transversely oblong quadrilateral plate of bone ; 

 its short lateral margin is joined by a linear 

 harmonia with the upper part of the os petro- 

 sum, on each side ; the wide anterior mar- 

 gin is similarly articulated with the single 

 parietal bone, and is slightly overlapped by its 

 posterior margin ; this representative of the 

 deltoid suture runs straight across the posterior 

 and upper part of the skull. 



Fig. 171. 



Occipital and sphenoidal cranial vertebrae, 

 setosa. ( Original.) 



* See L89<>ns d'Anatomie Comparee, Ed. 1837. 

 VOL. III. 



Occipital region of skull, Echidna setosa. (Original.) 



In the specimen in which the preceding 

 condition of the occipital vertebra was mani- 

 fested there was no trace of sagittal suture ; 

 the upper and middle region of the cra- 

 nium was covered by a single broad, slightly 

 convex, parietal bone, (Jig. 169, </,) joined 

 Echidna posteriorly, as above described, with the supra- 

 occipital, laterally with the petrous and sphe- 

 noid bones, and anteriorly with the sphenoid 

 and frontal bones, which the parietal overlaps 



2 B 



