896 H. H. NEWMAN AND J. THOMAS PATTERSON 
to suggest that it had been produced by a splitting of four scutes, 
but upon a closer inspection it will be seen that the upper row is 
closely associated with the left half of the band, and the lower 
row with the right half. In the light of the ‘theory of concres- 
cence’ one is tempted to suggest that it has been brought about 
through a failure of the embryonic primordium to affect a perfect 
meeting in this region, and consequently the inner ends of the 
two half-bands have slipped past each other for a short distance. 
In this connection attention should be called to a certain 
rather rare atypical condition which sometimes appears either in 
the ninth band or in the first row of the pelvic shield. The peculi- 
arity consists of a ‘jog’ at the middle point of the band or row, 
and in all probability has come about in a manner like that sug- 
gested just above. A specimen showing this condition in the first 
row of the pelvic shield is seen in fig. 23. 
The final ‘abnormality’ to be considered is shown in fig. 15. 
It really is a double peculiarity, in that both the eighth and ninth 
bands are involved. In the eighth band there are three small 
primary scutes lying just anterior to numbers 53-56 of the main 
scutes of the band. These small scutes do not affect the bony 
plates and must therefore be looked upon as very rudimentary 
in character. In the ninth band a somewhat similar ‘abnor- 
mality’ is seen, lying at scutes 52-56 of this band. It consists 
of four small scutes, which however affect the bony plates, as can 
be seen in the sketch of the under surface of this region of the 
band (fig. 15 6). It cannot be said to be a splitting because there 
are five plates in the lower or main row and only four above. 
In concluding this section of the paper two facts brought out 
in the foregoing account should receive especial attention, because 
of their direct bearing on what is to follow. (1) In all those 
cases of atypical variations that we have designated as bilateral 
there is a very strong tendency toward symmetry in the affected. 
regions both in position and extent. This is particularly clear 
in specimens classed as splittings, as the citation of a few cases 
will make evident: (a) In shell no. 8 band 1 has 57 scutes, and 
the two splittings occur in scutes 6-14 on the left and 44-51 on 
the right—were the right-hand split in scutes 44-52 instead of 
