30 Eobert E. Coker. 



the keel necessarily a single structure which we must not expect to 

 find divided 'i So far as I know, there is no phylogenetic evidence 

 of the neural series of turtles ever having been paired, or, of a double 

 dorsal keel in primitive turtles (cf. Newmann, '56, p. 92). If we 

 must regard these abnormalities as atavisms, we can hardly con- 

 ceive of the atavism taking so often the form of paired supernumer- 

 ary neurals; we would be led to assume, with jSTewman, that we 

 were presented with a modified longitudinal sequence. JSTow, disre- 

 garding atavism for the moment, we will consider the anomalies as 

 we find them. 



No case of unmistakably paired neurals has been observed in the 

 terrapins of North Carolina or Maryland, if we exclude the nuchal, 

 which is, however, usually included in the neural series; but there 

 are interesting abnormalities that are significant in this connection. 

 The nuchal, as has been seen, is sometimes paired, sometimes 

 marked by a median furrow in the position of the seam, and the 

 furrow may be so marked as to make it difficult to distinguish from 

 a seam, or there may be a seam on the anterior half of the scute, 

 continued posteriorly by a furrow (Thalassochelys, PL XIII, Fig. 89, 

 and PI. XIV, Fig. 94) . Apparently the difference between the furrow 

 and the seam is that the furrow divides incompletely, while the seam 

 divides the scute completely as far as the seam extends. Now in four 

 specimens of Malademmys (Nos. 73, 78, and 211 ; also No. 244 of 

 Table V), some of the neurals were marked by a median furrow 

 similar to the furrows observed on nuchals, and in each of these 

 cases the nuchal was either marked with a furrow, or paired. Espe- 

 cially suggestive is No. 70 (PI. I, Fig. 1) where N2 shows a short 

 furrow on the anterior rings,"^ and N4 has all of its rings intersected 

 anteriorly by a seam. Compare also PI. I, Fig. 2, of a terrapin 

 with nuchal paired and with furrows on Nl and N2. 



In logical sequence with these specimens come some terrapins 

 (M. littoralis) from Texas. In a small number of specimens placed 

 by Mr. W. P. Hay in the United States National Museum, we 

 observe all stages from very incomplete to complete division of the 



'Owing to the manner of reflection of ligtit the furrow on N2 is not appar- 

 ent in the photograph, though very evident in the shell. 



