Correspondence — Prof. T. G. Bonneij. 573 



its intermarginal scuta to a new genus, which I call Digerrhum. 

 The last two in their rudimental fifth pair of sternals, resemble 

 many PJeiirodira, and cannot be distinguished from the genus 

 Podocnemis now living in the Amazon." 



In this paragraph I have made the references adopted by Riitimeyer, 

 Lydekker and Boulenger, but I did not use the name Pleurosternum 

 for the PL Bullochii. Believing that tlie PL concinniim represented 

 that genus, I removed the PL Bullochii from it. It now remains to 

 ascertain the characters of the Pleurosternum concinnmn, since no 

 reference is made to it by the authors of the paper on which I am now 

 commenting, but who regard the PL Bidlockii as typical of the genus. 



M. Dollo well remarks (September Number) that the absence of 

 dermal sutures cannot alone place a genus of Tortoises in a separate 

 family from forms which possess such sutures. He regards 

 Erquilenesia as most resembling Eiiclastes, but not to belong to the 

 Propleuridse. But I think Euclastes is one of that group, and I 

 suspect that Dollo's characters of the skull define the group better 

 than the number of costal bones, which will however distinguish the 

 genera. E. D. Cope. 



ORIGIN OF CERTAIN BANDED GNEISSES. 

 Sir, — Mr. Teall's paper on the origin of certain banded gneisses 

 (p. 484) is a contribution of the utmost value to the discussion of a 

 very difficult subject. But, as I am responsible for another theory 

 concerning these gneisses, may I be allowed to say, that though I 

 admit the importance of the hypothesis, I still see great difficulties in- 

 the way of accepting it. These gneisses of the Lizard must not be 

 regarded alone ; a hypothesis which might alleviate our difficulties 

 here (and they are undoubtedly great) might increase them else- 

 where — and thus, did I assign an igneous origin to all the crystalline 

 rocks south of St. Keverne, I should find it difficult to know where 

 to stop in applying the explanation to other regions. Moreover, if 

 we are to explain these banded gneisses as the rolling or crushing^ 

 out of a complex of igneous rocks, not only must the flexibility of 

 the rocks have been very great, but also this complex originally must 

 have been a very intricate one. Now I have had a fairly large 

 experience in the habits of igneous rocks, and, so far as this goes, 

 such complications as would be required here are both rare in 

 occurrence and limited in extent. Yet at the Lizard the banded 

 series is of considerable thickness, and can be traced along the coast 

 for full two miles. I admit, however, that when I wrote my two 

 papers, I sometimes failed to distinguish between structures signifi- 

 cant of original constitution and those due to subsequent mechanical 

 action, for our knowledge of the latter is of very recent date ; but 

 to what extent I will not venture to say until I can again examine 

 the whole district. This I hope to do, but for various reasons must 

 defer the pleasure for a time. Probably it will be some years before 

 we can fairly determine the claims of conflicting hypotheses ; mean- 

 while it is well for science that Mr. Teall has advanced one with so 

 much moderation in statement, and clearness in reasoning. Perhaps 



