4 H. H. NEWMAN. 



the primary scutes are arranged in more or less regular transverse 

 rows, but these rows seldom run straight across the shield. 

 Instead it is commoner for a row that starts single on the margin 

 to become double for some distance where the bulge of the shell 

 is greatest, then to be single again toward the middle. Again a 

 row may start double laterally and be single for a short distance 

 toward the middle. Irregularities in scute rows are commonest 

 near the posterior border of the pectoral shield next to the 

 banded region. 



In the banded region the arrangement of scutes is quite 

 different. The bony plates are elongated antero-posteriorly to 

 form rows of units, in general appearance somewhat like the 

 keyboard of a piano. The posterior margin of the first band is 

 free from and slips over the anterior margin of the second, an 

 .arrangement that lends flexibility to the armor. The second 

 band bears a similar relation to the third and so throughout 

 the nine movable bands. Each bony plate is accompanied by 

 and partially covered by an elongated, wedge-shaped primary 

 scute. Figs, i and 3 represent two bands removed whole from 

 the armor. The typical band, however, is composed of an 

 even array of scutes arranged in a single row from margin to 

 margin. Only one third of one per cent, of the 16,200 bands 

 examined are in any way irregular in the arrangement of the 

 armor units. The rare exceptions, however, are of especial 

 interest and constitute the anomalies that are the subject of the 

 present study. These peculiar bands belong for the most part 

 to the types, the details of which are shown in Figs. I, 3 and 5. 

 They consist of bands that are partly single and partly double. 

 In Fig. I is seen a very common type of bilateral anomaly in 

 which a few scutes on each margin constitute the single part of 

 the band, while the main central region is double. In Fig. 3 

 we see an equally common condition in which the double part 

 of the band is confined to a number of scutes starting at some 

 distance from the margins but stopping short of the middle 

 portions of the band. Similar conditions of greater or less extent 

 are found unilaterally as frequently as bilaterally. Now these 

 irregular bands are in no sense abnormalities or products of 

 injury, but are of the same nature exactly as are the irregulari- 



