602 
. F. H. EDGEWORTH. 
the mandible. In stage 47 of Echidna the muscle has sepa- 
rated off and passes from the root of the crista parotica 
downwards and forwards to the outer surface of the mandible. 
This condition is also present in stage 50. In the adult 
Echidna this primary origin is lost, and the muscle arises 
from the ventral surface of the squamosal bone (Schulman). 
These phenomena support the view of Schulman that the 
detrahens mandibulEe is one of the lateral group of masticatory 
muscles. They negative the view of Toldt that it is split off 
from an already differentiated masseter, and suggestthat the 
muscle is, phylogenetically, more ancient than either the mas- 
seter, zygomatico-mandibularis or external pterygoid. 
The external pterygoid muscle is developed from the 
temporal in Dasyurus, pig and rabbit. Its origin in Orni- 
thorhynchus and Echidna was not determined owing to a want 
of the necessary stages. The process could be most clearly 
seen in Dasyurus, where the cells forming the muscle are at first 
small and non-striated, and probably proliferated from, and 
not formed by change of direction of, the longer cross-striated 
muscle cells of the temporal. In the rabbit and pig the for- 
mation of the external pterygoid takes place in an Anlage 
consisting of oval cells, and appeared to be due to separation 
of a portion of the temporal Anlage. 
The primary origin of the external pterygoid in Echidna is 
from the membrana obturatoria above the ala temporalis; in 
Dasyurus, and Didelphys aurita it is from the lower end of 
the lamina ascendens alae temporalis; in the rabbit and pig 
from the ala temporalis. On the formation of the alisphenoid 
bone outside the ala and its lamina ascendens, the muscle — in 
Dasyurus, Didelphys, rabbit and pig — arises from this bone ; 
and in the rabbit it gains an additional origin from the palate 
bone. In the adult Echidna (Schulman) the muscle arises 
from the planum infratemporale of the great wing of the 
sphenoid and from the ala temporalis palatini. 
Origin of the muscle from the alisphenoid bone is common : 
thus Lubosch describes the origin of the muscle, in Brady- 
pus from the pterygoid bone, in Dasypus and Tolypeutes by 
