Geo. S. Huntington . 159 
ing structures in the allied genera and species of the same zoological 
order. 
Determination, by reconstruction from these data, of the common plan 
of the muscle or muscle group underlying the specialized differentiations 
encountered in the individual forms; in other words the reconstruction 
of the primal antecedent myo-type of the order. 
Comparison, as extensive as possible or desirable, with the homologous 
structures in other mammalian orders. his is of especial importance 
where myological characters appear confined to limited groups within 
an order, not occurring in the remaining genera and species of the same 
order, but found again more or less widely distributed throughout other 
mammalian orders. 
Based on the preceding comparative study the attempt should be made 
to define the relation of the muscle or muscular variant to the common 
ancestral myological stem of the mammalian class. 
Conclusions respecting the phylogenetic value of the muscle or mus- 
cular group. 
The study of muscular variations, based on the data supplied by com- 
parative anatomy, shows that departures from the normal type of myo- 
logical development in any given form may be grouped, in reference to 
their derivation and significance, under one of the following three heads: 
1. Fortuitous variations. 2. Progressive variations. 3. Reversional 
variations. 
Fortuitous Variations——A very small percentage of the muscular ano- 
malies can, at least provisionally, be placed in this class. Congenital 
deficiencies of muscles or muscular groups, in part or in whole; maltfor- 
mations depending upon accidental errors in development, either by 
themselves or associated with cognate errors in the development of other 
structures; finally a small number of supernumerary muscles and muscu- 
lar slips. 
The arrangement of the Pectoral muscles shown in Pl. I, Fig. 1, may 
be taken as an example of this group of accidental or fortuitous varia- 
tions. 
The sheet of the Pectoralis major appears deficient in the portion of 
the sterno-costal segment corresponding to the third and fourth chondro- 
sternal junction. The muscular fibres which should fill the cleft are 
united to form an intermediate pectoral slip (Pectoralis minimus, var., 
tensor semivaginae articulationis humero-scapularis, Gruber), which 
crosses laterad between the Pectoralis major and minor to reach an ex- 
panded fascial insertion beneath the Deltoid close to the upper extremity 
of the humerus. 
