28 Kobert E. Coker. 



for example, the seam between the second and third shields of the 

 right costal series, meets M5, and this is the usual position of the 

 seam between normal Cl and C2. Considering this seam as cor- 

 responding to the normal seam between Cl and C2, we note that the 

 following costals (C3 to C5 in this shell) have exactly the relations 

 to the marginals normal for costals 2, 3 and 4. We have only to 

 assume that nonnal Cl is represented by two scutes in this carapace, 

 or that one of the first two scutes is supernumerary ; the right costal 

 series is thus made perfectly intelligible hj considering its relations 

 to the marginal series, and disregarding its relations to the neural 

 series. Turning to the left costal series, the four large scutes pre- 

 sent a perfectly normal appearance, and have nearly normal rela- 

 tions to the lateral apices of the neural series ; yet not one of the 

 four has normal relations to the marginals, though the marginal 

 series of each side is nearly normal. This series would be intelligible 

 if we disregarded the marginal relations. Either series is inter- 

 pretable, provided we do not apply the same criteria to both sides. 

 It is clear that the exact identification of supernumerary scutes must 

 often be impossible or quite arbitrary. The significance of such 

 cases will become clearer after the following section. 



Neurals. 



Excluding the luichal, for present pui-poses, the neural series con- 

 sists typically of a row of five median scutes, which dovetail later- 

 ally with the costals. The first four usually bear marked prom- 

 inences, which together form a low^ serrated keel. The fifth, some- 

 times, especially in younger specimens, shows a very low and incon- 

 spicuous continuation of the keel (cf. PI. YI, Figs. 12 and 13). 

 This keel is suggestive of the serrated dorsal keel of the tail of 

 Chelydra, and Hay and ISTewmann regard the neural scutes as seri- 

 ally homologous with the scutes of the dorsal keel of the tail of 

 Chelydra. In view of the accredited phylogenetic significance of the 

 keel prominences they may aid in the interpretation of supernumer- 

 ary scutes in the neural series. Such scutes are almost always placed 

 not symmetrically, in longitudinal sequence, or alongside of each 

 other, but asymmetrically and wedged in together. The photographs 



