Diversity in the Scutes of Chelonia. 25 



Typically, each costal series consists of four scutes, which dove- 

 tail between the five neurals. The seam between CI and C2 meets 

 the fifth marginal near its anterior end, and the succeeding costal 

 seams meet alternate marginals (seventh, ninth and eleventh). 

 These typical relations of the costals to the shields of the other 

 series, may aid in the diagnosis of supernumerary schutes (v. Fig. A). 



In Fig. 9 the first large scute in the L. costal series shows essen- 

 tially the relations of the normal' first costal, so that the small scute 

 anterior to it may fairly be considered the supernumerary element. 

 Sush anterior supernumerary costals may occur on both sides, 

 approximately symmetrically (IsTos. 244, 245, and 246, PI. I, Fig. 

 2, and PL VII, Figs. 18 and 19) ; or on both sides but quite asjan- 

 metrically (No. 218, PI. IX, Fig. 41), or on only one side (No. 9, 

 right side, PI. II, Fig. 3, and Nos. 4 and 131, left side, PI. VII, Fig. 

 20, and PI. V, Fig. 9). In each case referred to above, the extra 

 scute was small in comparison with the other costals. In No. 218 

 supernumerary scutes were observed both anteriorly and posteriorly 

 (Figs. 41 and 44). 



Perhaps some of the cases to be discussed below belong in one of 

 the above classes. In Nos. 210 (Figs. 5 and 17) and 151 (Figs. 7 

 and 15) the supernumerary scute in the costal series may be the 

 large CI ; but it would be difficult if not impossible to determine 

 in these instances whether the first or second scute is to be regarded 

 as supernumerary. We must always recognize the possibility that 

 in some cases we are not presented with the four normal scutes as 

 individuals plus an extra, "supernumerary," individual, but simply 

 with a five-scute series instead of a typical four-scute series. Fur- 

 thermore it may be quite immaterial whether an anomalous scute is 

 termed a neural or a costal ; the designation may be without real 

 significance. Of course, however, if supernumerary scutes are attrib- 

 uted a morphological value, their proper classification is of the first 

 importance (cf. below, p. 42). 



The difficulty of identification of scutes is enhanced in the pos- 

 terior region, where it is often difficult to decide whether a given 

 anomalous scute is to be classed as a neural or a costal [cf. Nos. 210 

 (Figs. 5 and 17) and 252, Fig. 49]. It was found necessary to 

 adopt certain rules of classification which may be' in 'some measure 



