THE MANDIIUJLAR AND HYOID :\rUSCLES OF MAi\[MALS. 637 
appear impossible. Similarly, they cannot be homologous 
with the constrictor colli o£ Sauropsida, for this is developed 
as an extension backwards of the levator hyoidei and inter- 
hyoideus,nor with the backward extension of the inter-hyoideus 
which occurs in many Amphibia. 
It is therefore doubtful whether these skin muscles of 
Mammals have any homologies in non-mammals ; they have 
probably been developed within the Mammalian phylum. 
The phenomena recorded above suggest (1) that the 
sphincter profundus, platysma, and sphincter superhcialis 
muscles have been developed within the Mammalian phylum. 
(2) That the other muscles developed in the hyoid segment 
are derived from (a) a levator hyoidei, arising from the outer 
surface of the auditory capsule and inserted into the upper part 
of the hyoid bar ; (b) a ventral constrictor or inter-hyoideus, 
arising from the hyoid bar and passing to a median raphe. 
When the primitive conditions of the mandibular and hyoid 
muscles in Mammalia are compared with those of Amphibia 
and Sauropsida, it becomes clear that they point to the same 
conclusion as that arrived at by Fiirbringer in his ^‘^Abstam- 
mnno- der Sauo-ethiere — that Mammals are descended from 
O 
a pro-amphibian stock. 
I have, in conclusion, to offer many thanks to Prof. J. P. 
Hill for the loan of sections of Dasy urus viverrimus and 
Oruithorhynchus, to Prof. W. N. Parker for the loan of 
sections of an adult Echidna, to Dr. Assheton for the loan of 
specimens of Echidna (stages 47 and 50) and Phascolarctos, 
to Herbert Donnison, Esq., for the head of a Ph oca vitulina, 
to Fraulein Snethlage for an embryo and head of an adult 
Brady pus marmoratus, and to the Director of the Calcutta 
Museum for the head of a Man is pen tadacty la. 
The expenses of the work and its publication have been 
defrayed by a grant from the Committee of the Bristol E^ni- 
versity Colston Society. 
Septem l>er 2iul, 1913. 
