Diversity iu tlie Scutes of Chelonia. 63 



with tiie second to iiftli margiiials, the narrowness of the nuchal 

 bringing the lirst marginal into a position anterior to the iirst neural. 

 Where an anterior supernumerary costal occurs in Malaclemmys, 

 the relation of the hrst two costals to the marginal corresponds to 

 that in Tlialassochelys, Fig. 9, left. Sometimes the small first costal 

 of llialassochelys is wanting (seven instances) and then the condition 

 approximates that of Malaclemmys (cf. Figs. 57 and 92). Sym- 

 metry in this abnormality was never noted; in fact, in two of the 

 seven specimens a supernumerary scute was present on the opposite 

 side, Figs. 52 and 64. This suggests that the absence of this scute 

 is attributable to some primary asymmetiy rather than to reversion. 



Another not infrequent abnormality of costals consists in the pres- 

 ence of two scutes in the place of C5. The two scutes, together, 

 have practically the usual relations of C5 to marginals on the one 

 side and to neurals on the other. Cf. Figs. 70, 83, right, and 56, 

 82, 88, left. Nos. 10, 66, 115, 116, 117, 118, 114 left (Figs. 52, 

 101) have the same abnonnality. Contrast wdth these the posterior 

 supernumerary scute of ±^o. 69^ Fig. 76, and 114, right. Fig. 101, 

 also the incomplete division of LC5 in iSTo. 42 (Fig. 69). 



The abnormalities of costals so far mentioned, while manifesting 

 asymmetry so far as the numbers of scutes on the two sides goes, 

 leave the tw-o series symmetrical in general plan. But the plan is 

 disarranged in such sj^x'cimens as ^STos. 94 (Fig. 90) and 97 (Fig. 

 91) and, as we shall see below, the number of extra neurals occurring 

 in association with such abnormalities of costals depends on the 

 extent of the asymmetiy. 



Asymmetry may occur without supernumerary costals. Figs. 71, 

 72, etc.). On the other hand, supernumerary costals may occur 

 symmetrically; Fig. 78 illustrates this, if it be assumed, as seems 

 apparent, that LCI is wanting while the next to last costal of each 

 side is supernumerary, 



Neurals. 



Paired neurals or neurals incompletely divided in the median line 

 were not found, but a slight median furrow Avas sometimes observed. 



A very symmetrical supernumerary neural sometimes appears 

 in sea-turtles just posterior to ]Sr4. This scute is almost perfectly 



