594 
F. H. EDGEWOETH. 
angulare of the lower jaw^ which are present in Amphibia and 
Reptiles. This opinion was adopted by Gaupp, who homo- 
logisedthe tympanic bone with the angulare^ and pointed our- 
that the presence of a pterygo-tympanicns in some Mammals 
was an additional fact in favour of van Kampen^s theory. 
It is probable, then, that the muscle — on the occurrence of 
ossification — arose from the (Mammalian) pterygoid bone, and 
was inserted into the rneckelian (perhaps the only) portion of 
a tympanic bone which formed a covering bone for the old 
jaw with an incudo-ineckelian joint. On the development of 
the new squamoso-ma,ndibular joint, the muscle, being inserted 
behind this point, lost its significance as regards any action on 
the jaw, and as far as present information goes, this condition 
is not preserved in any adult Mammal. 
No young enough Edentate embryos have been investigated 
to ascertain whether, in ontogenetic development, the muscle 
is inserted into the rneckelian portion of the tympanic bone, 
but an indication of at least its phylogenetic existence is given 
by the temporary insertion of the muscle in Dasyurus into 
Meckel’s cartilage opposite this portion of the bone. In late 
eml^ryos of Dasyurus n o v e m c i n c t a and B r a d y p u s m a r - 
moratus the muscle is inserted into the later formed lower 
limb of the bone. 
Insertion solely into the tympanic bone is preserved to the 
adult condition in Brady pus tri dactyl us; the muscle 
generally spreads to neighbouring structures. Thus Schulman 
described its insertion in Ornithorhynchus to the junction of 
the os sphenoideum and os petrosum, in Choloepus to the liga- 
mentumaccessorium mediate and walls of the fissura squamoso- 
petro-tympanica, in Manis to the ‘^M’rommelhohle,” in 
Tamandua to the bone round the cleft through which the 
chorda tympani passes ; and Kostanecki described the insertion 
in Dasyurus sexcinctus to the bulla tympanica and os 
sphenoides. 
The origin of the pterygo-tympanicus from the pterygoid 
bone is, similarly, rarely preserved ; in Edentates only, as far 
as present information goes, in Tamandua (Schulman), 
