Diversity in the Scutes of Chelonia. 61 



3. Review of Obsekvations. 

 Marginals. 



In strong contradistinction to most turtles there is no definite 

 number of marginals that can be considered normal. In the 12- 

 plan (see above, p. 49), the first costal is in contact with only two 

 marginals (Ml and M2), the second with four (M's 2, 3, 4 and 5) — 

 Fig. 74. In the 13-plan, the first costal is in contact with three mar- 

 ginals, the second with M's 3, 4, 5 and 6. The differences be- 

 tween the two is therefore, the presence in the one case, the absence 

 in the others, of a scute between Ml and tlie marginal opposite the 

 seam posterior to CI (marked Mx in Figs. 76 and 79). This 

 scute varies in size from a very minute scute entirely surrounded 

 by the scutes preceding and following it (Fig. 76) to a size as large 

 as the others, Fig. 79 right. Compare Fig. 79 left, Fig. 76 right, 

 and Fig. 83, left and right. Sometimes, with the 12-plan anteriorly, 

 there occurs a posterior scute, making the number really 13. This 

 is spoken of as the 12-plan with a supernumerary scute posteriorly. 



Sometimes an extra marginal appears in the region of C2. This 

 scute, 3Iy, Fig. 76, leads to a 14-plan if Mx is also present. Again, 

 a scute, Mz, may intervene between the most posterior marginal and 

 the marginal opposite the seam posterior to nonnal C5. In this 

 event the series contains 13, 14, or 15 scutes, according as the 

 12-plan, 13-plan, or 14-plan prevails anteriorly. Compare Fig. 92 

 right— 13 scutes (Mz), Fig. 79,' right— 14 scutes {3fx, Mz), Fig. 79, 

 left — 15 scutes (Mx, My, Mz). 



As to the relative frequency of the common marginal plans, 12-12 

 and 13-13 occur approximately in equal proportions, 12-13 and 13-12 

 occur each about one-third as often as either of the symmetrical 

 plans. The scutes My and Mz are of rare occurrence. Supernumer- 

 ary scutes at other places were not observed. The stippled area of 

 MIO, Fig. 84, represents an infolded area. The posterior limb is 

 folded over the carapace in this region, and the slight malformity 

 of the margin noted in this region in this and other specimens 

 (Figs. 80 and. 76) undoubtedly results from undue pressure of the 

 limb against the margin, and this suggests the same explanation for 

 the malformed margins of Ma.laclemmys (Figs. 32, 37, 39, 40). 



